Method and device for transmitting and receiving downlink control channel for controlling inter-cell interference in wireless communication system

ABSTRACT

The present invention relates to a wireless communication system, and more particularly, to a method and device for transmitting and receiving a downlink control channel for controlling inter-cell interference in a wireless communication system. An embodiment of the present invention provides a method for transmitting a downlink control channel from a base station to a terminal, and the method may comprise: a step of determining whether a downlink subframe is of a first type or a second type; and a step in which if the downlink subframe is of the first type, the number of OFDM symbols for transmitting the downlink control channel is set to a preset value (N), and the downlink control channel is transmitted using N OFDM symbols.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/884,387, filed on May 9, 2013, now U.S. Pat. No. 9,144,069, which isthe National Stage filing under 35 U.S.C. 371 of InternationalApplication No. PCT/KR2011/008543, filed on Nov. 10, 2011, and alsoclaims the benefit of U.S. Provisional Application No. 61/412,797, filedon Nov. 12, 2010, the contents of all of which are hereby incorporatedby reference herein in their entireties.

FIELD OF THE INVENTION

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for transceiving a downlinkcontrol channel for inter-cell interference coordination in a wirelesscommunication system.

DESCRIPTION OF THE RELATED ART

FIG. 1 is a diagram for a heterogeneous network wireless communicationsystem 100 including a macro base station and a micro base station. Inthis disclosure, a terminology named a heterogeneous network means anetwork having a macro base station 110 and micro base stations 121 and122 coexist therein despite using the same RAT (radio accesstechnology).

The macro base station 110 has a wide coverage and a high transmissionpower and means a general base station in a wireless communicationsystem. The macro base station 110 may be called a macro cell.

Each of the micro base stations 121 and 122 may be called such a name asa micro cell, a pico cell, a femto cell, a home eNB (HeNB), a relay andthe like. Each of the micro base stations 121 and 122 is a small-scaleversion of the macro base station 110 and can independently operate byperforming most of the functions of the macro base station 110. And,each of the micro base stations 121 and 122 may include a base stationof an overlay type that is installed within an area covered by the macrobase station. Moreover, each of the micro base stations 121 and 122 mayinclude a base station of a non-overlay type that can be installed in aradio shadow area not covered by the macro base station. Each of themicro base stations 121 and 122 can accommodate a smaller number of userequipments with a coverage and transmission power smaller than those ofthe macro base station 110.

One user equipment 131 may be directly served by the macro base station110 [hereinafter such a user equipment shall be called a macro userequipment (macro-UE)]. Another user equipment 132 may be served by themicro base station 122 [hereinafter such a user equipment shall becalled a micro user equipment (micro-UE)]. Occasionally, the userequipment 132 existing within the coverage of the micro base station 122may be served by the macro base station 110.

Micro base stations may be classified into two kinds of types dependingon whether restriction is put on an access by a user equipment. Afirstfirst type corresponds to a CSG (closed subscriber group) microbase station. And, a second type corresponds to an OSC (open subscribergroup) micro base station. The CSG micro base station can serve specificaccess-granted user equipments only, while the OSG micro base stationcan serve all user equipments without separate access restriction.

Meanwhile, PDCCH (physical downlink control channel) is the channel thatcarries control information provided to a user equipment by a basestation. Information indicating that PDCCH is carried on a prescribednumber of time units (e.g., OFDM (orthogonal frequency divisionmultiplexing) symbol(s), etc.) in a downlink subframe can be provided toa user equipment by a base station through PCFICH (physical controlformat indicator channel).

SUMMARY OF THE INVENTION

When a user equipment served by a macro base station in a heterogeneousnetwork mentioned in the foregoing description is located close to amicro base station, a strong downlink (DL) signal from the micro basestation may cause interference on a DL signal received from a macro basestation by a macro user equipment. And, a DL signal from a macro basestation may make storing interference with a user equipment served by amicro base station. Similarly, inter-cell interference may be generatedin uplink. To prevent this inter-cell interference, it is able toconsider a method of preventing interference on an interference-givencell (hereinafter named a victim cell) by blocking a transmission of aninterference-giving cell (hereinafter named an aggressor cell) on someresource elements.

Thus, even if the inter-cell interference coordinating scheme isapplied, it is unable to block a transmission of a prescribed referencesignal (e.g., a cell-specific reference signal (CRS)) essential to userequipments served by an aggressor cell. Hence, the reference signal ofthe aggressor cell still causes interference to a victim cell.Particularly, even if the inter-cell interference coordinating scheme isapplied, it may happen that an aggressor cell causes interference toPCFICH of a victim cell, whereby performance of PDCCH decoding in a userequipment of the victim cell may be considerably lowered. If the PDCCHdecoding performance is degraded, UL/DL (uplink/downlink) scheduling isnot correctly performed, whereby overall performance of a network can beconsiderably lowered.

The technical task of the present invention is to provide a method oftransceiving a downlink control channel (particularly, PCFICH, PDCCH,etc.) accurately and efficiently with respect to inter-cell interferencecoordination.

Technical tasks obtainable from the present invention are non-limitedthe above-mentioned technical task. And, other unmentioned technicaltasks can be clearly understood from the following description by thosehaving ordinary skill in the technical field to which the presentinvention pertains.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, a method oftransmitting a downlink control channel, which is transmitted to a userequipment by a base station, according to one embodiment of the presentinvention may include the steps of determining whether a downlinksubframe corresponds to a firstfirst type subframe or a second typesubframe, setting the number of OFDM (orthogonal frequency divisionmultiplexing) symbols used for a transmission of the downlink controlchannel to a predefined number (N) if the downlink subframe correspondsto the firstfirst type subframe, and transmitting the downlink controlchannel on the OFDM symbols amounting to a number equal to the N.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, a base station, which transmits adownlink control channel, according to another embodiment of the presentinvention may include a receiving module configured to receive an uplinksignal from a user equipment, a transmitting module configured totransmit a downlink signal to the user equipment, and a processorconfigured to control the base station including the receiving moduleand the transmitting module, the processor configured to determinewhether a downlink subframe corresponds to a firstfirst type subframe ora second type subframe, the processor configured to set the number ofOFDM (orthogonal frequency division multiplexing) symbols used for atransmission of the downlink control channel to a predefined number (N)if the downlink subframe corresponds to the firstfirst type subframe,the processor configured to transmit the downlink control channel on theOFDM symbols amounting to a number equal to the N.

The following matters may be applicable in common to the above-mentionedembodiments of the present invention.

Preferably, information on the number of the OFDM symbols used for thetransmission of the downlink control channel may not be transmitted inthe downlink subframe corresponding to the firstfirst type subframe.

Preferably, if the downlink subframe corresponds to the second typesubframe, the base station may transmit information on the number of theOFDM symbols used for the transmission of the downlink control channelvia PCFICH (physical control format indicator channel) and may transmitthe downlink control channel on the OFDM symbols amounting the a numberequal to the information transmitted via the PCFICH.

Preferably, the firstfirst type subframe may include a subframe havingan inter-cell interference coordination performed thereon by aneighboring cell and the second type subframe may include a subframehaving the inter-cell interference coordination not performed thereon bythe neighboring cell. More preferably, the inter-cell interferencecoordination performed subframe may include the subframe configured bythe neighboring cell as one of ABS (almost blank subframe), MBSFN(multicast broadcast single frequency network) subframe orABS-with-MBSFN subframe.

Preferably, the downlink control channel may include a PDCCH (physicaldownlink control channel).

To further achieve these and other advantages and in accordance with thepurpose of the present invention, a method of receiving a downlinkcontrol signal, which is received by a user equipment from a basestation, according to a further embodiment of the present invention mayinclude the steps of measuring a power of a signal received from each ofa serving cell and a neighboring cell, calculating a difference valueresulting from subtracting the power of the signal received from theneighboring cell from the power of the signal received from the servingcell, comparing the difference value to a prescribed reference value, ifthe difference value is equal to or smaller than the prescribedreference value, assuming that the number of OFDM (orthogonal frequencydivision multiplexing) symbols used for a transmission of the downlinkcontrol channel from the serving cell as a predefined value (N), anddecoding the downlink control channel on the OFDM symbols amounting to anumber equal to the N.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, a user equipment, which receives adownlink control channel, according to another further embodiment of thepresent invention may include a receiving module configured to receive adownlink signal from a base station, a transmitting module configured totransmit an uplink signal to the base station, and a processorconfigured to control the user equipment including the receiving moduleand the transmitting module, the processor configured to measure a powerof a signal received from each of a serving cell and a neighboring cell,the processor configured to calculate a difference value resulting fromsubtracting the power of the signal received from the neighboring cellfrom the power of the signal received from the serving cell, theprocessor configured to compare the difference value to a prescribedreference value, the processor configured to assume that the number ofOFDM (orthogonal frequency division multiplexing) symbols used for atransmission of the downlink control channel from the serving cell as apredefined value (N) if the difference value is equal to or smaller thanthe prescribed reference value, the processor configured to decode thedownlink control channel on the OFDM symbols amounting to a number equalto the N.

The following matters may be applicable in common to the above-mentionedembodiments of the present invention.

Preferably, if the difference value is greater than the prescribedreference value, the user equipment obtains information on the number ofthe OFDM symbols used for the transmission of the downlink controlchannel by decoding PCFICH (physical control format indicator channel)from the serving cell and may decode the downlink control channel on theOFDM symbols amounting to a number equal to the information obtained viathe PCFICH.

Preferably, the prescribed reference value may be determined based on anSINR (signal-to-interference plus noise ratio) meeting a decodingcondition of the PCFICH from the serving cell. Preferably, theprescribed reference value may be determined based on a reception powerdifference value for determining whether to operate a handover. Morepreferably, the prescribed reference value may be set to either a valuehigher than a reception power difference value for determining whetherto operate a handover into the neighboring cell from the serving cell ina range expansion area of the serving cell or a value lower than areception power difference value for determining whether to operate thehandover into the neighboring cell from the serving cell in a rangeexpansion area of the neighboring cell

Preferably, a result of measuring the power of the signal received fromeach of the serving cell and the neighboring cell may be reported to thebase station.

Preferably, the downlink control channel may include a PDCCH (physicaldownlink control channel).

The above-mentioned general description of the present invention and thefollowing details of the present invention are exemplary and may beprovided for the additional description of the invention disclosed inclaims.

According to the present invention, a method of transceiving a downlinkcontrol channel (particularly, PCFICH, PDCCH, etc.) accurately andefficiently with respect to inter-cell interference coordination can beprovided.

Effects obtainable from the present invention are non-limited by theabove mentioned effect. And, other unmentioned effects can be clearlyunderstood from the following description by those having ordinary skillin the technical field to which the present invention pertains.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention.

FIG. 1 is a diagram for a heterogeneous network wireless communicationsystem.

FIG. 2 is a diagram for a structure of a downlink radio frame.

FIG. 3 is a diagram of a resource grid in a downlink slot.

FIG. 4 is a diagram for one example of a structure of a downlinksubframe.

FIG. 5 is a diagram for a structure of an uplink subframe.

FIG. 6 is a diagram for a configuration of a wireless communicationsystem having multiple antennas.

FIG. 7 is a diagram of CRS and DRS patterns defined in the legacy 3GPPLTE system.

FIG. 8 is a diagram of an uplink subframe structure including SRSsymbols.

FIG. 9 is a diagram for one example of transceiving unit functionimplementation of FDD mode relay node.

FIG. 10 is a diagram to describe a transmission from a relay node to auser equipment and a downlink transmission from a base station to a userequipment.

FIG. 11 is a diagram for one example of ABS application.

FIG. 12 is a diagram to describe PCFICH mapped resource elements.

FIG. 13 is a diagram to illustrate interference caused to PCFICH of avictim cell in case of applying eICIC scheme.

FIG. 14 is a flowchart to describe an operation of a base stationaccording to the present invention.

FIG. 15 is a flowchart to describe an operation of a user equipmentaccording to the present invention.

FIG. 16 is a diagram to describe one embodiment of the present inventionin a heterogeneous network between a macro cell and a pico cell.

FIG. 17 is a diagram to describe another embodiment of the presentinvention in a heterogeneous network between a macro cell and a picocell.

FIG. 18 is a diagram to describe one embodiment of the present inventionin a heterogeneous network between a macro cell and a femto cell.

FIG. 19 is a diagram for configurations of a base station device 1910and a user equipment device 1920 according to one preferred embodimentof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First of all, the following embodiments correspond to combinations ofelements and features of the present invention in prescribed forms. And,the respective elements or features may be considered as selectiveunless they are explicitly mentioned. Each of the elements or featurescan be implemented in a form failing to be combined with other elementsor features. Moreover, an embodiment of the present invention may beimplemented by combining elements and/or features together in part. Asequence of operations explained for each embodiment of the presentinvention may be modifiable. Some configurations or features of oneembodiment may be included in another embodiment or substituted withcorresponding configurations or features of another embodiment.

In the present specification, embodiments of the present invention aredescribed centering on the data transmission/reception relations betweena base station and a terminal. In this case, the base station may bemeaningful as a terminal node of a network which directly performscommunication with the terminal. In this disclosure, a specificoperation explained as performed by a base station may be performed byan upper node of the base station in some cases.

In particular, in a network constructed with a plurality of networknodes including a base station, it is apparent that various operationsperformed for communication with a terminal can be performed by a basestation or other networks other than the base station. ‘Base station(BS)’ may be substituted with such a terminology as a fixed station, aNode B, an eNode B (eNB), an access point (AP) and the like. A relay maybe substituted with such a terminology as a relay node (RN), a relaystation (RS) and the like. And, ‘terminal’ may be substituted with sucha terminology as a user equipment (UE), a mobile station (MS), a mobilesubscriber station (MSS), a subscriber station (SS) and the like.

Specific terminologies used for the following description may beprovided to help the understanding of the present invention. And, theuse of the specific terminology may be modified into another form withinthe scope of the technical idea of the present invention.

Occasionally, to prevent the present invention from getting vaguer,structures and/or devices known to the public may be skipped orrepresented as block diagrams centering on the core functions of thestructures and/or devices. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like partsin this specification.

Embodiments of the present invention may be supported by the disclosedstandard documents of at least one of wireless access systems includingIEEE 802 system, 3GPP system, 3GPP LTE and LTE-A (LTE-Advanced) systemand 3GPP2 system. In particular, the steps or parts, which are notexplained to clearly reveal the technical idea of the present invention,in the embodiments of the present invention may be supported by theabove documents. Moreover, all terminologies disclosed in this documentmay be supported by the above standard documents.

The following description of embodiments of the present invention mayapply to various wireless access systems including CDMA (code divisionmultiple access), FDMA (frequency division multiple access), TDMA (timedivision multiple access), OFDMA (orthogonal frequency division multipleaccess), SC-FDMA (single carrier frequency division multiple access) andthe like. CDMA can be implemented with such a radio technology as UTRA(universal terrestrial radio access), CDMA 2000 and the like. TDMA canbe implemented with such a radio technology as GSM/GPRS/EDGE (GlobalSystem for Mobile communications)/General Packet Radio Service/EnhancedData Rates for GSM Evolution). OFDMA can be implemented with such aradio technology as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE802.20, E-UTRA (Evolved UTRA), etc. UTRA is a part of UMTS (UniversalMobile Telecommunications System). 3GPP (3rd Generation PartnershipProject) LTE (long term evolution) is a part of E-UMTS (Evolved UMTS)that uses E-UTRA. The 3GPP LTE adopts OFDMA in downlink (hereinafterabbreviated) DL and SC-FDMA in uplink (hereinafter abbreviated UL). And,LTE-A (LTE-Advanced) is an evolved version of 3GPP LTE. WiMAX may beexplained by IEEE 802.16e standard (e.g., WirelessMAN-OFDMA referencesystem) and advanced IEEE 802.16m standard (e.g., WirelessMAN-OFDMAadvanced system). For clarity, the following description mainly concerns3GPP LTE system or 3GPP LTE-A system, by which the technical idea of thepresent invention may be non-limited.

A structure of a downlink (DL) radio frame is described with referenceto FIG. 2 as follows.

In a cellular OFDM radio packet communication system, UL/DL(uplink/downlink) data packet transmission is performed by a unit ofsubframe. And, one subframe is defined as a predetermined time intervalincluding a plurality of OFDM symbols. In the 3GPP LTE standard, atype-1 radio frame structure applicable to FDD (frequency divisionduplex) and a type-2 radio frame structure applicable to TDD (timedivision duplex) are supported.

FIG. 2 (a) is a diagram for a structure of a downlink radio frame oftype 1. A DL (downlink) radio frame includes 10 subframes. Each of thesubframes includes 2 slots. And, a time taken to transmit one subframeis defined as a transmission time interval (hereinafter abbreviatedTTI). For instance, one subframe may have a length of 1 ms and one slotmay have a length of 0.5 ms. One slot may include a plurality of OFDMsymbols in time domain or may include a plurality of resource blocks(RBs) in frequency domain. Since 3GPP system uses OFDMA in downlink,OFDM symbol indicates one symbol duration. The OFDM symbol may be namedSC-FDMA symbol or symbol duration. Resource block (RB) is a resourceallocation unit and may include a plurality of contiguous subcarriers inone slot.

The number of OFDM symbols included in one slot may vary in accordancewith a configuration of CP. The CP may be categorized into an extendedCP and a normal CP. For instance, in case that OFDM symbols areconfigured by the normal CP, the number of OFDM symbols included in oneslot may be 7. In case that OFDM symbols are configured by the extendedCP, since a length of one OFDM symbol increases, the number of OFDMsymbols included in one slot may be smaller than that of the case of thenormal CP. In case of the extended CP, for instance, the number of OFDMsymbols included in one slot may be 6. If a channel status is unstable(e.g., a UE is moving at high speed), it may be able to use the extendedCP to further reduce the inter-symbol interference.

When a normal CP is used, since one slot includes 7 OFDM symbols, onesubframe includes 14 OFDM symbols. In this case, first 2 or 3 OFDMsymbols of each subframe may be allocated to PDCCH (physical downlinkcontrol channel), while the rest of the OFDM symbols are allocated toPDSCH (physical downlink shared channel).

FIG. 2 (b) is a diagram for a structure of a downlink radio frame oftype 2. A type-2 radio frame includes 2 half frames. Each of the halfframe includes 5 subframes, DwPTS (downlink pilot time slot), GP (guardperiod) and UpPTS (uplink pilot time slot). And, one of the subframesincludes 2 slots. The DwPTS is used for initial cell search,synchronization or channel estimation in a user equipment. The UpPTS isused for channel estimation in a base station and uplink transmissionsynchronization of a user equipment. The guard period is a period foreliminating interference generated in uplink due to multi-path delay ofa downlink signal between uplink and downlink. Meanwhile, one subframeis constructed with 2 slots irrespective of a type of a radio frame.

The above-described structures of the radio frame are just exemplary.And, the number of subframes included in a radio frame, the number ofslots included in the subframe and the number of symbols included in theslot may be modified in various ways.

FIG. 3 is a diagram for one example of a resource grid in a downlink(DL) slot. In the drawing, one DL slot may include 7 OFDM symbols intime domain and one resource block (RB) may include 12 subcarriers infrequency domain, by which the present invention may be non-limited. Forinstance, in case of a normal CP (cyclic prefix), one slot includes 7OFDM symbols. Yet, in case of an extended CP (extended-CP), one slot mayinclude 6 OFDM symbols. Each element on a resource grid may be called aresource element (RE). One resource block includes 12×7 resourceelements. N^(DL) indicates the number of resource blocks included in aDL slot. And, the value of the N^(DL) may depend on a DL transmissionbandwidth. A structure of UL slot may be identical to that of the DLslot.

FIG. 4 is a diagram for a structure of a downlink (DL) subframe. Maximum3 OFDM symbols situated in a head part of a first slot of one subframecorrespond to a control region to which a control channel is allocated.The rest of OFDM symbols correspond to a data region to which PDSCH(physical downlink shared channel) is allocated. DL control channelsused by 3GPP LTE system may include PCFICH (Physical Control FormatIndicator Channel), PDCCH (Physical Downlink Control Channel), PHICH(Physical hybrid automatic repeat request indicator Channel) and thelike. The PCFICH is transmitted in a first OFDM symbol of a subframe andincludes information on the number of OFDM symbols used for atransmission of a control channel within the subframe. The PHICHincludes HARQ ACK/NACK signal in response to a UL transmission. Controlinformation carried on PDCCH may be called downlink control information(DCI). The DCI may include UL or DL scheduling information or a ULtransmission power control command for a random UE (user equipment)group. The PDCCH may include transmission format and resource allocationinformation of DL-SCH (downlink shared channel), resource allocationinformation on UL-SCH (uplink shared channel), paging information on PCH(paging channel), system information on DL-SCH, resource allocation ofsuch an upper layer control message as a random access responsetransmitted on PDSCH, transmission power control command set forindividual UEs within a random UE group, transmission power controlinformation, activation of VoIP (voice over IP) and the like. Aplurality of PDCCHs can be transmitted within the control region. A userequipment can monitor a plurality of the PDCCHs. The PDCCH istransmitted as an aggregation of at least one or more contiguous CCEs(control channel elements). The CCE is a logical allocation unit used toprovide the PDCCH at a coding rate based on a radio channel status. TheCCE may correspond to a plurality of REGs (resource element groups). Aformat of the PDCCH and the number of available PDCCH bits may bedetermined in accordance with correlation between the number of CCEs anda coding rate provided by the CCE. A base station determines a PDCCHformat in accordance with a DCI which is to be transmitted to a userequipment and attaches a CRC (cyclic redundancy check) to controlinformation. The CRC is masked with an identifier named RNTI (radionetwork temporary identifier) in accordance with an owner or usage ofthe PDCCH. For instance, if the PDCCH is provided for a specific userequipment, the CRC may be masked with an identifier (e.g., cell-RNTI(C-RNTI)) of the corresponding user equipment. In case that the PDCCH isprovided for a paging message, the CRC may be masked with a pagingindicator identifier (e.g., P-RNTI). If the PDCCH is provided for systeminformation (particularly, for a system information block (SIC)), theCRC may be masked with a system information identifier and a systeminformation RNTI (SI-RNTI). In order to indicate a random accessresponse to a transmission of a random access preamble of a userequipment, the CRC may be masked with RA-RNTI (random access-RNTI).

FIG. 5 is a diagram for a structure of an uplink (UL) subframe. A ULsubframe may be divided into a control region and a data region infrequency domain. A physical UL control channel (PUCCH) including ULcontrol information may be allocated to the control region. And, aphysical UL shared channel (PUSCH) including user data may be allocatedto the data region. In order to maintain single carrier property, oneuser equipment does not transmit PUCCH and PUSCH simultaneously. PUCCHfor one user equipment may be allocated to a resource block pair (RBpair) in subframe. Resource blocks belonging to the resource block pairmay occupy different subcarriers for 2 slots. Namely, a resource blockpair allocated to PUCCH is frequency-hopped on a slot boundary.

Modeling of Multi-Antenna (MIMO) System

FIG. 6 is a diagram for a configuration of a wireless communicationsystem including multiple antennas.

Referring to FIG. 6 (a), if the number of transmitting antennas isincremented into N_(T) and the number of receiving antennas isincremented into N_(R), theoretical channel transmission capacity isincreased in proportion to the number of antennas unlike the case that atransmitter or receiver uses a plurality of antennas. Hence, atransmission rate may be enhanced and frequency efficiency may beremarkably raised. The transmission rate according to the increase ofthe channel transmission capacity may be theoretically raised by anamount resulting from multiplying a maximum transmission rate R₀ of thecase of using a single antenna by a rate increasing rate R_(i).R _(i)=min(N _(T) ,N _(R))  [Formula 1]

For instance, in an MIMO communication system, which uses 4 transmittingantennas and 4 receiving antennas, it may be able to obtain atransmission rate 4 times higher than that of a single antenna system.After this theoretical capacity increase of the MIMO system has beenproved in the middle of the 90's, many efforts are ongoing to be made tovarious techniques for drive it into substantial data rate improvement.Some of these techniques are already adopted as standards for variouswireless communications such as 3G mobile communications, a nextgeneration wireless LAN and the like.

The trends for the MIMO relevant studies are explained as follows. Firstof all, many ongoing efforts are made in various aspects to develop andresearch information theory study relevant to MIMO communicationcapacity calculations and the like in various channel configurations andmultiple access environments, radio channel measurement and modelderivation study for MIMO systems, spatiotemporal signal processingtechnique study for transmission reliability enhancement andtransmission rate improvement and the like.

In order to explain a communicating method in an MIMO system in detail,mathematical modeling can be represented as follows. Assume that N_(T)transmitting antennas and N_(R) receiving antennas exist in this system.

First of all, a transmission signal is explained. If there are N_(T)transmitting antennas, N_(T) maximum transmittable informations exist.Hence, the transmission information may be represented as follows.s=[s ₁ ,s ₂ , . . . ,s _(N) _(T) ]^(T)  [Formula 2]

Meanwhile, transmission power can be set different for each transmissioninformation s₁, s₂, . . . , s_(N) _(T) . If the respective transmissionpowers are set to P₁, P₂, . . . , P_(N) _(T) , the transmission poweradjusted transmission information may be represented as follows.ŝ=[ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T) =[P ₁ s ₁ ,P ₂ s ₂ , . . . , P_(N) _(T) s _(N) _(T) ]^(T)   [Formula 3]

And, Ŝ may be represented as follows using a transmission power diagonalmatrix P.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {P\; s}}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$

If a weight matrix W is applied to the transmission power adjustedtransmission information vector ŝ, a case of configuring N_(T)transmission signals x₁, x₂, . . . , x_(N) _(T) actually transmitted canbe taken into consideration as follows. In this case, the weight matrixW plays a role in properly distributing the transmission information toeach antenna according to a transmission channel status and the like.The x₁, x₂, . . . , x_(N) _(T) may be represented using a vector X asfollows.

$\begin{matrix}{x = {\quad{\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{t} \\\vdots \\x_{N_{T}}\end{bmatrix} = {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{21} & w_{22} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{i\; N_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {W\; P\; s}}}}}} & \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack\end{matrix}$

In Formula 5, w_(ij) indicates a weight between an i^(th) transmittingantenna and j^(th) information. And, W may be called a precoding matrix.

When N_(R) receiving antennas exist, if reception signals of thereceiving antennas are set to y₁, y₂, . . . , y_(N) _(R) , a receptionsignal vector can be represented as follows.y=[y ₁ ,y ₂ , . . . ,y _(N) _(R) ]^(T)  [Formula 6]

If a channel is modeled in MIMO wireless communication system, thechannel can be represented as an index of a transmitting antenna and anindex of a receiving antenna. A channel between a transmitting antenna jand a receiving antenna i may be represented as h_(ij). In the h_(ij),it should be noted that a receiving antenna index is followed by atransmitting antenna index in order of index.

FIG. 6 (b) shows a channel to a receiving antenna i from each of N_(T)transmitting antennas. These channels may be represented as a vector ormatrix in a manner of tying the channels b together. Referring to FIG. 6(b), the channels between the receiving antenna i and the N_(T)transmitting antennas can be represented as follows.h _(i) ^(T) =[h _(i1) ,h _(i2) , . . . ,h _(iN) _(T) ]  [Formula 7]

Hence, al the channels arriving from N_(T) transmitting antennas toN_(R) relieving antennas may be expressed as follows.

$\begin{matrix}{H = {\quad{\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}}} & \left\lbrack {{Formula}\mspace{14mu} 8} \right\rbrack\end{matrix}$

In an actual channel, a transmission signal passes through a channelmatrix H and then has AWGN (additive white Gaussian noise) addedthereto. If white noses n₁, n₂, . . . , n_(N) _(R) respectively added toN_(R) receiving antennas, the white noises n₁, n₂, . . . , n_(N) _(R)can be represented as follows.n=[n ₁ ,n ₂ , . . . ,n _(N) _(R) ]^(T)  [Formula 9]

Hence, the reception signal vector may be expressed as follows throughthe above-mentioned formula modeling.

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {{{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{i\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \mspace{455mu}\begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}} = {{H\; x} + n}}}} & \left\lbrack {{Formula}\mspace{14mu} 10} \right\rbrack\end{matrix}$

Meanwhile, the number of rows/columns of a channel matrix H indicating achannel state is dependent on the number of transmitting/receivingantennas. The number of rows in the channel matrix H is equal to thenumber N_(R) of the receiving antennas. The number of columns in thechannel matrix H is equal to the number N_(T) of the transmittingantennas. In particular, the channel matrix H becomes N_(R)×N_(T)matrix.

A rank of matrix is defined as a minimum one of the number ofindependent rows and the number of independent columns. Hence, it may beimpossible for a rank of matrix to become greater than the number ofrows or columns. A rank (rank (H)) of a channel matrix H is restrictedto the following.Rank(H)≦min(N _(T) ,N _(R))  [Formula 11]

For another definition of a rank, when Eigen value decomposition isperformed on a matrix, a rank may be defined as the number of Eigenvalues except 0. Similarly, for a further definition of a rank, whensingular value decomposition is performed, a rank may be defined as thenumber of singular values except 0. Hence, the physical meaning of arank in a channel matrix may be regarded as a maximum number for sendingdifferent informations on a given channel.

Reference Signal (RS)

When a packet is transmitted in a wireless communication system, sincethe transmitted packet is transmitted on a radio channel, signaldistortion may occur in the course of the transmission. In order for areceiving side to correctly receive the distorted signal, distortion ina received signal should be corrected using channel information. Inorder to acquire the channel information, after a signal known to both areceiving side and a transmitting side has been transmitted, the channelinformation can be acquired with a degree of distortion on receiving thesignal on a channel. This signal may be called a pilot signal or areference signal.

In case of transmitting or receiving data using MIMO, a channel statusbetween each transmitting antenna and each receiving antenna should beobtained to receive a correct signal. Hence, a separate reference signalneeds to be present for each transmitting antenna.

Downlink reference signals may include a common reference signal (CRS)shared with all user equipments in a cell and a dedicated referencesignal (DRS) for a specific user equipment only. By these referencesignals, information for channel estimation and demodulation can beprovided.

A receiving side (e.g., user equipment) estimates a state of a channelfrom CRS and may be able to feed back such an indicator related to achannel quality as CQI (Channel Quality Indicator), PMI (PrecodingMatrix Index) and RI (Rank Indicator) to a transmitting side (e.g., basestation). The CRS may be called a cell-specific reference signal. An RSrelated to feedback of such channel state information (CSI) asCQI/PMI/RI can be separately defined as CSI-RS.

Meanwhile, DRS may be transmitted on a corresponding RE if demodulationof data on PDSCH is necessary. A user equipment many be informed of apresence or non-presence of DRS by an upper layer. In particular, theuser may be informed that the DRS is valid only if the correspondingPDSCH is mapped. The DRS may be called a UE-specific reference signal ora demodulation reference signal (DMRS).

FIG. 7 is a diagram to illustrate a pattern in which CRS and DRS definedby the legacy 3GPP LTE system (e.g., Release-8) are mapped on a downlinkresource block. The downlink resource block, which is a unit for mappinga reference signal, may be represented as a unit of ‘1 subframe ontime×12 subcarriers on frequency’. In particular, one resource block mayhave a length of 14 OFDM symbols on time in case of a normal CP [FIG. 7(a)] or a length of 12 OFDM symbols in case of an extended CP [FIG. 7(b)].

FIG. 7 shows a position of a reference signal on a resource block in asystem having a base station support 4 transmitting antennas. In FIG. 7,resource elements (REs) denoted by 0, 1, 2 and 3 indicate positions ofCRS for antenna port indexes 0, 1, 2 and 3, respectively. Meanwhile, aresource element denoted by ‘D’ in FIG. 7 indicates a position of DRS.

In the following description, CRS is explained in detail.

First of all, CRS is used to estimate a channel of a physical antennastage. The CRS is a reference signal receivable in common by all userequipments (UEs) in a cell and is distributed over a whole band. The CRSmay be used for the purpose of channel state information (CSI)acquisition and data demodulation.

The CRS may be defined in various forms in accordance with antennaconfiguration. 3GPP LTE (e.g., Release-8) system supports variousantenna configurations and a downlink signal transmitting side (e.g.,base station) may have three kinds of antenna configurations including asingle antenna, 2 transmitting antennas, 4 transmitting antennas and thelike. In case that a base station performs a single antennatransmission, a reference signal for a single antenna port is arranged.In case that a base station performs 2-antenna transmission, referencesignals for 2 antenna ports are arranged by time division multiplexingand/or frequency division multiplexing. In particular, the referencesignals for 2 antenna ports are arranged on different time resourcesand/or different frequency resources to be discriminated from eachother. In case that a base station performs 4-antenna transmission,reference signals for 4 antenna ports are arranged by TDM/FDM. Channelinformation estimated via CRS by a downlink signal receiving side (e.g.,user equipment) may be used for demodulation of data transmitted by sucha transmission scheme as Single Antenna Transmission, Transmitdiversity, Closed-loop Spatial multiplexing, Open-loop Spatialmultiplexing, Multi-User MIMO (MU-MIMO) and the like.

In case that MIMO is supported, when a reference signal is transmittedfrom a prescribed antenna port, a reference signal is carried at aresource element (RE) position designated by a reference signal patternbut no signal is carried at a resource element (RE) position designatedfor another antenna port.

A rule for mapping CRS on a resource block follows Formula 12.

$\begin{matrix}{{k = {{6\; m} + {\left( {v + v_{shift}} \right)\mspace{11mu}{mod}\mspace{14mu} 6}}}{l = \left\{ {{{\begin{matrix}{0,{N_{symb}^{DL} - 3}} & {{{if}\mspace{14mu} p} \in \left\{ {0,1} \right\}} \\1 & {{{if}\mspace{14mu} p} \in \left\{ {2,3} \right\}}\end{matrix}m} = 0},1,\ldots\;,{{{2 \cdot N_{R\; B}^{D\; L}} - {1m^{\prime}}} = {{m + N_{R\; B}^{\max,{D\; L}} - {N_{R\; B}^{D\; L}v}} = \left\{ {{\begin{matrix}0 & {{{if}\mspace{14mu} p} = {{0\mspace{14mu}{and}\mspace{14mu} l} = 0}} \\3 & {{{if}\mspace{14mu} p} = {{0\mspace{14mu}{and}\mspace{14mu} l} \neq 0}} \\3 & {{{if}\mspace{14mu} p} = {{1\mspace{14mu}{and}\mspace{14mu} l} = 0}} \\0 & {{{if}\mspace{14mu} p} = {{1\mspace{14mu}{and}\mspace{14mu} l} \neq 0}} \\{3\left( {n_{s}\mspace{11mu}{mod}\mspace{14mu} 2} \right)} & {{{if}\mspace{14mu} p} = 2} \\{3 + {3\left( {n_{s}\mspace{11mu}{mod}\mspace{11mu} 2} \right)}} & {{{if}\mspace{14mu} p} = 3}\end{matrix}v_{shift}} = {N_{ID}^{cell}\mspace{14mu}{mod}\mspace{14mu} 6}} \right.}}} \right.}} & \left\lbrack {{Formula}\mspace{14mu} 12} \right\rbrack\end{matrix}$

In Formula 12, k indicates a subcarrier index, l indicates a symbolindex, and p indicates an antenna port index. N_(symb) ^(DL) indicatesthe number of OFDM symbols of one downlink slot, N_(RB) ^(DL) indicatesthe number of resource blocks allocated to downlink, n_(s) indicates aslot index, and N_(ID) ^(cell) indicates a cell ID. ‘mod’ means a modulooperation. A position of a reference signal in frequency domain dependson a value of V_(shift). Since the V_(shift) value depends on a cell IDas well, a position of a reference signal has a frequency shift valuedifferent per cell.

In particular, a position in frequency domain may be set to differ bybeing shifted in order to raise channel estimation performance throughCRS. For instance, if a reference signal is situated at every 3subcarriers, a prescribed cell enables the reference signal to bearranged on a subcarrier of 3k and another cell enables the referencesignal to be arranged on a subcarrier of 3k+1. In viewpoint of oneantenna port, a reference signal is arranged by 6-RE interval (i.e.,6-subcarrier interval) in frequency domain and maintains 3-RE intervalin frequency domain from an RE on which a reference signal for anotherantenna port is arranged.

For the CRS, power boosting may be applicable. In this case, the powerboosting means that a reference signal is transmitted with higher powerin a manner of bringing power not from an RE allocated for the referencesignal but from another RE among resource elements (REs) of one OFDMsymbol.

A reference signal position in time domain is arranged by apredetermined interval by setting symbol index (l) 0 of each slot to astart point. A time interval is defined different in accordance with aCP length. In case of a normal CP, a reference signal is situated at asymbol index 0 of a slot and a reference signal is situated at a symbolindex 4 of the slot. Reference signals for maximum 2 antenna ports aredefined on one OFDM symbol. Hence, in case of 4-transmitting antennatransmission, reference signals for antenna ports 0 and 1 are situatedat symbol indexes 0 and 4 (or symbol indexes 0 and 3 in case of anextended CP) of a slot, respectively and reference signals for antennaports 2 and 3 are situated at symbol index 1 of the slot. Yet, frequencypositions of the reference signals for the antenna ports 2 and 3 may beswitched to each other in a second slot.

In order to support spectral efficiency higher than that of theconventional 3GPP LTE (e.g., Release-8) system, it is able to design asystem (e.g., LTE-A) system having an extended antenna configuration.For instance, the extended antenna configuration may include an8-transmitting antenna configuration. In the system having the extendedantenna configuration, it may be necessary to support user equipmentsoperating in the conventional antenna configuration. Namely, it may benecessary to support backward compatibility. Hence, it may be necessaryto support a reference signal pattern according to the conventionalantenna configuration and it may be necessary to design a new referencesignal pattern for an additional antenna configuration. In this case, ifCRS for a new antenna port is added to a system having a conventionalantenna configuration, it is disadvantageous in that a reference signaloverhead rapidly increases to lower a data rate. In consideration ofthis matter, a separate reference signal (CSI-RS) for a channel stateinformation (CSI) measurement for the new antenna port may be introducedinto LTE-A (LTE-advanced) system evolved from 3GPP LTE.

In the following description, DRS is explained in detail.

First of all, DRS (or UE-specific reference signal) is a referencesignal used for data demodulation. When MIMO transmission is performed,a precoding weight used for a specific user equipment in MIMOtransmission is used for a reference signal as it is. Hence, when a userequipment receives a reference signal, it is able to estimate anequivalent channel having a transmission channel combined with theprecoding weight transmitted from each transmitting antenna.

The conventional 3GPP LTE system (e.g., Release-8) supports maximum4-transmitting antenna transmission and DRS for rank 1 beamforming isdefined. The DRS for the rank 1 beamforming may be represented as areference signal for antenna port index 5. A rule for mapping DRS on aresource block may follow Formula 13 and Formula 14. Formula 13 relatesto a normal CP, while Formula 14 relates to an extended CP.

$\begin{matrix}{{k = {{\left( k^{\prime} \right)\mspace{11mu}{mod}\mspace{14mu} N_{sc}^{RB}} + {N_{sc}^{RB} \cdot n_{PRB}}}}{k^{\prime} = \left\{ {{\begin{matrix}{{4m^{\prime}} + v_{shift}} & {{{if}\mspace{14mu} l} \in \left\{ {2,3} \right\}} \\{{4m^{\prime}} + {\left( {2 + v_{shift}} \right)\mspace{14mu}{mod}\mspace{14mu} 4}} & {{{if}\mspace{14mu} l} \in \left\{ {5,6} \right\}}\end{matrix} l} = \left\{ {{\begin{matrix}3 & {l^{\prime} = 0} \\6 & {l^{\prime} = 1} \\2 & {l^{\prime} = 2} \\5 & {l^{\prime} = 3}\end{matrix}l^{\prime}} = \left\{ {{{\begin{matrix}{0,1} & {{{if}\mspace{14mu} n_{s}\mspace{11mu}{mod}\mspace{14mu} 2} = 0} \\{2,3} & {{{if}\mspace{14mu} n_{s}\mspace{14mu}{mod}\mspace{14mu} 2} = 1}\end{matrix}m^{\prime}} = 0},1,\ldots\;,{{{3N_{RB}^{PDSCH}} - {1v_{shift}}} = {N_{ID}^{cell}\mspace{14mu}{mod}\mspace{14mu} 3}}} \right.} \right.} \right.}} & \left\lbrack {{Formula}\mspace{14mu} 13} \right\rbrack \\{{k = {{\left( k^{\prime} \right)\mspace{11mu}{mod}\mspace{14mu} N_{sc}^{RB}} + {N_{sc}^{RB} \cdot n_{PRB}}}}k^{\prime} = \left\{ {{\begin{matrix}{{3m^{\prime}} + v_{shift}} & {{{if}\mspace{14mu} l} = 4} \\{{3m^{\prime}} + {\left( {2 + v_{shift}} \right)\mspace{14mu}{mod}\mspace{14mu} 3}} & {{{if}\mspace{14mu} l} = 1}\end{matrix}l} = \left\{ {{\begin{matrix}4 & {l^{\prime} \in \left\{ {0,2} \right\}} \\1 & {l^{\prime} = 1}\end{matrix}l^{\prime}} = \left\{ {{{\begin{matrix}0 & {{{if}\mspace{14mu} n_{s}\mspace{11mu}{mod}\mspace{14mu} 2} = 0} \\{1,2} & {{{if}\mspace{14mu} n_{s}\mspace{14mu}{mod}\mspace{14mu} 2} = 1}\end{matrix}m^{\prime}} = 0},1,\ldots\;,{{{4N_{RB}^{PDSCH}} - {1v_{shift}}} = {N_{ID}^{cell}\mspace{14mu}{mod}\mspace{14mu} 3}}} \right.} \right.} \right.} & \left\lbrack {{Formula}\mspace{14mu} 14} \right\rbrack\end{matrix}$

In Formula 13 and Formula 14, k indicates a subcarrier index, lindicates a symbol index, and p indicates an antenna port index. N_(SC)^(RB) indicates a resource block size in frequency domain and may berepresented as the number of subcarriers. n_(PRB) indicates a physicalresource block number. And, N_(RB) ^(PDSCH) indicates a bandwidth of aresource block of a corresponding PDSCH transmission. n_(s) indicates aslot index and N_(ID) ^(cell) indicates a cell ID. Moreover, ‘mod’ meansa modulo operation. A position of a reference signal in frequency domaindepends on a value of V_(shift). Since the V_(shift) value depends on acell ID as well, a position of a reference signal has a frequency shiftvalue different per cell.

Meanwhile, in a system of LTE-A (LTE-Advanced) evolved from 3GPP LTE,MIMO of high order, multi-cell transmission, advanced MU-MIMO and thelike are taken into consideration. In order to support efficientreference signal management and advanced transmission scheme, DRS baseddata demodulation is taken in to consideration. In particular, asidefrom DRS (antenna port index 5) for the rank 1 beamforming defined bythe conventional 3GPP LTE (e.g., Release-8), in order to support datatransmission via an added antenna, it is able to define DRS for at leasttwo layers.

Cooperative Multi-Point (CoMP)

In accordance with the advanced system performance requirements of 3GPPLTE-A system, CoMP transmission/reception scheme (represented as one ofco-MIMO (collaborative MIMO), network MIMO, etc.) has been proposed. TheCoMP technology can enhance performance of a user equipment located at acell edge and increase average sector throughput as well.

Generally, in a multi-cell environment having a frequency reuse factorset to 1, the performance and average sector throughput of the userequipment located at the cell edge may be lowered due to inter-cellinterference (ICI). In order to reduce the ICI, a conventional LTEsystem has applied a method of providing an appropriate throughputperformance to a user equipment located at a cell edge in an environmentrestricted by interference using a simple passive scheme such as FFR(fractional frequency reuse) via UE-specific power control and the like.Yet, reducing the ICI or reusing the ICI as a signal desired by a userequipment may be more preferable than lowering a frequency resource useper cell. To achieve this object, CoMP transmission schemes may beapplicable.

The CoMP schemes applicable to a DL case may be mainly classified intojoint processing (JP) scheme and coordinated scheduling/coordinatedbeamforming (CS/CB) scheme.

The JP scheme can use data at each point (e.g., base station) of CoMPcooperation unit. And, the CoMP cooperation unit may mean a set of basestations used for the cooperative transmission scheme. Moreover, the JPscheme may be classified into a joint transmission scheme and a dynamiccell selection scheme.

The joint transmission scheme means a scheme of transmitting PDSCH froma plurality of points (portion or all of CoMP cooperation unit) at atime. In particular, data transmitted to a single user equipment may besimultaneously from a plurality of transmission points. According to thejoint transmission scheme, a quality of a coherently or non-coherentlyreceived signal can be improved and interference on another userequipment can be actively eliminated.

The dynamic cell selection scheme means the scheme of transmitting PDSCHfrom one point (of CoMP cooperation unit) at a time. In particular, datatransmitted to a single user equipment at a specific timing point istransmitted from one point, the rest of points in the cooperation unitat that timing point do not perform data transmission to thecorresponding user equipment, and a point of transmitting data to thecorresponding user equipment may be dynamically selected.

According to the CS/CB scheme, CoMP cooperation units can cooperativelyperform beamforming of data transmission to a single user equipment. Inthis case, although the data is transmitted from a serving cell only,user scheduling/beamforming may be determined by the coordination ofcells of the corresponding CoMP cooperation unit.

Meanwhile, in case of uplink, coordinated multi-point reception meansthat a signal transmitted by coordination of a plurality of pointsgeographically spaced apart from each other is received. CoMP schemesapplicable to a case of uplink may be classified into joint reception(JR) and coordinated scheduling/coordinated beamforming (CS/CB).

The JR scheme means that a signal transmitted on PUSCH is received by aplurality of reception points. And, the CS/CB scheme means that userscheduling/beamforming is determined by coordination of cells of CoMPcooperation unit despite that PUSCH is received by one point only.

Sounding Reference Signal (SRS)

Sounding reference signal (SRS) is mainly used for a base station toperform a frequency-selective scheduling in UL by performing a channelquality measurement and is not associated with UL data and/or controlinformation transmission, by which the sounding reference signal isnon-limited. And, the SRS may be usable for the purpose of an improvedpower control or for the purpose of supporting various start-upfunctions of recently unscheduled user equipments. For example, thestart-up functions may include an initial modulation and coding scheme(MCS), an initial power control for data transmission, a timing advanceand frequency half-selective scheduling (e.g., a scheduling performed ina manner that a frequency resource is selectively allocated in afirstfirst slot of a subframe but that a frequency resourcepseudo-randomly hops into another frequency in a second slot of thesubframe), and the like.

The SRS may be usable for a DL channel quality measurement on theassumption that a radio channel is reciprocal between UL and DL. Thisassumption is particularly effective to a TDD (time division duplex)system in which a UL and a DL share the same frequency band with eachother but are discriminated from each other in time domain.

A subframe, in which SRS is transmitted by a random user equipmentwithin a cell, is indicated by cell-specific broadcast signaling. A4-bit cell-specific parameter ‘srsSubframeConfiguration’ indicates 15kinds of available configurations of a subframe for transmitting SRSwithin each radio frame. By this configuration, flexibility foradjusting an SRS overhead in accordance with a network arrangementscenario can be provided. A configuration of a remaining one (16^(th))of the parameter is to completely switch off an SRS transmission withina cell and may be suitable for a cell that mainly serves fast userequipments for example.

Referring to FIG. 8, SRS is always transmitted on a last SC-FDMA symbolof a configured subframe. Hence, SRS and DMRS (demodulation referencesignal) are located on different SC-FDMA symbols, respectively. PUSCHdata transmission is not allowed to be performed on SC-FDMA symboldesignated to SRS transmission. Hence, if a sounding overhead is highest(i.e., a case that an SRS transmission symbol exists in every subframe),it does not exceed about 7%.

Each SRS symbol is generated for a given time unit and frequency band bya basic sequence (e.g., a random sequence, a set of ZC-based (ZadoffChu-based) sequences) and every user equipment within a cell uses thesame basic sequence. In doing so, SRS transmissions from a plurality ofuser equipments within a cell on the same frequency band can beorthogonally identified by different cyclic shifts of the basic sequenceassigned to a plurality of the user equipments, respectively. Althoughan SRS sequence of a different cell may be identifiable by assigning adifferent basic sequence to each cell, orthogonality between thedifferent basic sequences are not guaranteed.

Relay Node

A relay node can be taken into consideration for an expansion of fastdata rate coverage, an enhancement of group mobility, a temporarynetwork arrangement, an enhancement of a cell boundary throughput,and/or a network coverage offering to a new area. A relay node mayinclude a fixed relay node located at a fixed place or a mobile relaynode having a moving location.

A relay node plays a role in forwarding transmission and receptionbetween a base station and a user equipment. And, two kinds of links(i.e., a backhaul link and an access link) differing from each other inattributes are applied to carrier frequency bands, respectively. Thebase station may include a donor cell. And, the relay node is connectedby wireless to a wireless-access network via the donor cell.

In case that a backhaul link between the base station and the relay nodeuses a DL frequency band or DL subframe resource, it may be representedas a backhaul downlink. In case that a backhaul link between the basestation and the relay node uses a UL frequency band or UL subframeresource, it may be represented as a backhaul uplink. In this case, thefrequency band is a resource allocated in FDD (frequency divisionduplex) mode and the subframe is a resource allocated in TDD (timedivision duplex) mode. Similarly, in case that an access link betweenthe relay node and the user equipment(s) uses a DL frequency band or DLsubframe resource, it may be represented as an access downlink. In casethat an access link between the relay node and the user equipment(s)uses a UL frequency band or UL subframe resource, it may be representedas an access uplink.

A UL reception function and a DL transmission function are necessary fora base station and a UL transmission function and a DL receptionfunction are necessary for a user equipment. On the other hand, afunction of a backhaul UL transmission to a base station, a function ofan access UL reception from a user equipment, a function of a backhaulDL reception from a base station and a function of an access DLtransmission to a user equipment are necessary for a relay node.

FIG. 9 is a diagram for one example of transceiving unit functionimplementation of an FDD mode relay node. A reception function of arelay node is conceptionally explained as follows. First of all, a DLsignal received from a base station is forwarded to an FTT (fast Fouriertransform) module 912 via a duplexer 911 and an OFDMA baseband receptionprocess 913 is then performed. A UL signal received from a userequipment is forwarded to an FFT module 922 via a duplexer 921 and aDFT-s-OFDMA (discrete Fourier transform-spread-OFDMA) baseband receptionprocess 923 is then performed. The process for receiving a DL signalfrom a base station and the process for receiving a UL signal from auser equipment may be simultaneously performed in parallel with eachother. On the other hand, a transmission function of the relay node isconceptionally explained as follows. First of all, a UL signaltransmitted to a base station is transmitted via a DFT-s-OFDMA basebandtransmission process 933, an IFFT (inverse FFT) module 932 and aduplexer 931. The process for transmitting the UL signal to the basestation and the process for transmitting the DL signal to the userequipment may be simultaneously performed in parallel with each other.And, the duplexers shown in one direction may be implemented into asingle bidirectional duplexer. For instance, the duplexer 911 and theduplexer 931 may be implemented into a single bidirectional duplexer.For another instance, the duplexer 921 and the duplexer 941 may beimplemented into a single bidirectional duplexer. In particular, thebidirectional duplexer may be implemented in a manner that an IFFTmodule and baseband process module line associated withtransmission/reception on a specific carrier frequency band divergesfrom a single bidirectional duplexer.

Meanwhile, regarding a use of a band (or spectrum) of a relay node, onecase in which a backhaul link operates on the same frequency band of anaccess link may be referred to as ‘in-band’, and the other case in whicha backhaul link operates on a frequency band different from that of anaccess link is referred to as ‘out-band’. In both of the above-mentionedtwo cases of the in-band and the out-band, it is necessary for a userequipment (hereinafter named a legacy user equipment) operating in alegacy LTE system (e.g., Release-8) to access a donor cell.

Relay nodes may be classified into a transparent relay node and anon-transparent relay node by depending on whether a user equipmentrecognizes the relay node. In particular, the ‘transparent’ may mean acase that a user equipment is unable to recognize whether the userequipment is communicating with a network through a relay node. And, the‘non-transparent’ may mean a case that a user equipment is able torecognize whether the user equipment is communicating with a networkthrough a relay node.

Regarding controls of a relay node, relay nodes may be classified intoone relay node configured as a part of a donor cell and another relaynode capable of controlling a cell by itself.

Although the relay node configured as a part of the donor cell may havea relay node identifier (ID), the relay node does not have a cellidentity of its own. If at least one portion of RPM (Radio ResourceManagement) is controlled by a base station having the donor cell belongthereto (despite that the rest of the RPM is located at the relay node),the above-mentioned relay node may be considered as a relay node (RN)configured as a part of the donor cell. Preferably, this relay node maybe able to support a legacy user equipment. For example, smartrepeaters, decode-and-forward relays, a variety of L2 (second layer)relay nodes, and a type-2 relay node may belong to the category of theabove-mentioned relay node.

Regarding a relay node configured to self-control a cell, this relaynode controls one or more cells, a unique physical layer cell identityis provided to each cell controlled by the relay node, and the same RPMmechanism may be usable. In aspect of a user equipment, there is nodifference between accessing a cell controlled by a relay node andaccessing a cell controlled by a general base station. Preferably, acell controlled by the above-mentioned relay node may be able to supporta legacy user equipment. For example, a self-backhauling RN, an L3(third layer) relay node, a type-1 relay node, and a type-1a relay nodemay belong to the category of the above-mentioned relay node.

The type-1 relay node plays a role as an in-band relay node incontrolling a plurality of cells, and a user equipment may consider eachof the cells as a separate cell discriminated from a donor cell.Moreover, each of a plurality of the cells has a physical cell ID(defined in LTE Release-8) of its own and the relay node may be able totransmit a synchronization channel of its own, a reference signal andthe like. In case of a single-cell operation, a user equipment maydirectly receive scheduling information and HARQ feedback from a relaynode and can transmit a control channel (scheduling request (SR), CQI,ACK/NACK, etc.) of its own to a relay node. Moreover, legacy userequipments (e.g., user equipments operating in LTE Release-8 system) mayconsider the type-1 relay node as a legacy base station (e.g., a basestation operating in the LTE Release-8 system). In particular, thetype-1 relay node has backward compatibility. Meanwhile, in aspect ofuser equipments operating in LTE-A system, the type-1 relay node isconsidered as a base station different from a legacy base station,whereby performance thereof can be enhanced.

The type-1a relay node operates in the out-band, and has the samefeatures as those of the type-1 relay node. Operation of the type-1 arelay node can be configured to minimize (or eliminate) the influence onL1 (firstfirst layer) operation.

The type-2 relay node corresponds to an in-band relay node but has noseparate physical cell ID not to form a new cell. The type-2 relay nodeis transparent to a legacy user equipment and the legacy user equipmentis unable to recognize the presence of the type-2 relay node. Althoughthe type-2 relay node is able to transmit PDSCH, it does not transmitCRS and PDCCH at least.

Meanwhile, in order for a relay node to operate in in-band, prescribedresources in time-frequency space must be reserved for a backhaul linkand these resources may be configured not be used for an access link.This configuration is called ‘resource partitioning’.

The general principles related to the resource partitioning in a relaynode may be described as follows. First of all, a backhaul downlink andan access downlink may be multiplexed together on a single carrierfrequency by Time Division Multiplexing (TDM) [i.e., either the backhauldownlink or the access downlink is activated in specific time.).Similarly, a backhaul uplink and an access uplink may be multiplexedtogether on a single carrier frequency by TDM [i.e., either the backhauluplink or the access uplink can be activated in specific time).

Regarding the backhaul link multiplexing by FDD, a backhaul downlinktransmission is performed on a downlink frequency band, and a backhauluplink transmission is performed on an uplink frequency band. Regardingthe backhaul link multiplexing by TDD, a backhaul downlink transmissionis performed in a downlink subframe of a base station or a relay node,and a backhaul uplink transmission is performed in an uplink subframe ofthe base station or the relay node.

In case of an in-band relay mode, for example, provided that both abackhaul downlink reception from a base station and an access downlinktransmission to a user equipment are simultaneously performed on aprescribed frequency band, a signal transmitted from a transmitting endof a relay node may be received by a receiving end of the relay node,whereby signal interference or RF jamming may occur at an RF front-endof the relay node. Similarly, if both an access uplink reception from auser equipment and a backhaul uplink transmission to a base station aresimultaneously performed on a prescribed frequency band, signalinterference may occur at the RF front-end of the relay node. Therefore,it may be difficult to implement the simultaneous transmission andreception on a single frequency band at a relay node unless a sufficientseparation between a received signal and a transmitted signal isprovided [e.g., a transmitting antenna and a receiving antenna areinstalled in a manner of being sufficiently spaced apart from each other(e.g., installed on/under the ground)].

As a solution for the above signal interference problem, it may be ableto enable a relay node not to transmit a signal to a user equipmentwhile receiving a signal from a donor cell. In particular, a gap isgenerated in a transmission from the relay node to the user equipment,and the user equipment (e.g., a legacy user equipment, etc.) may beconfigured not to expect any transmission from the relay node duringthis gap. The above-mentioned gap may be generated by constructing MBSFN(Multicast Broadcast Single Frequency Network) subframe [cf. FIG. 10].Referring to FIG. 10, in a first subframe 1010 that is a normalsubframe, a downlink (i.e., an access downlink) control signal and dataare transmitted from a relay node to a user equipment. In a secondsubframe 1020 that is an MBSFN subframe, while a control signal istransmitted from the relay node to the user equipment on a controlregion 1021 of a downlink subframe but any transmission from the relaynode to the user equipment is not performed in the rest region 1022 ofthe downlink subframe. In doing so, since a legacy user equipmentexpects a transmission of physical downlink control channel (PDCCH) inall downlink subframes (i.e., the relay node needs to support legacyuser equipments within a coverage of the relay node to receive PDCCH ineach subframe and to perform a measurement function thereof), it isnecessary for the PDCCH to be transmitted in all the downlink subframesin order for each legacy user equipment to operate correctly. Therefore,in a subframe (i.e., the second subframe) configured for a downlink(i.e., backhaul downlink) transmission from a base station to a relaynode, the relay node needs to perform an access downlink transmission infirst N OFDM symbols (N=1, 2 or 3) rather than to receive a backhauldownlink. For this, since PDCCH is transmitted from the relay node tothe user equipment in a control region 1021 of the second subframe, itmay be able to provide backward compatibility with a legacy userequipment served by the relay node. While no signal is transmitted inthe rest region 1022 of the second subframe from the relay node, therelay node may be able to receive a transmission from the base station.Therefore, the above-mentioned resource partitioning scheme can preventthe access downlink transmission and the backhaul downlink receptionfrom being simultaneously performed by the in-band relay node.

The second subframe 1022, which uses the MBSFN subframe, shall bedescribed in detail as follows. First of all, a control region 1021 ofthe second subframe may be referred to as a relay node non-hearinginterval. In particular, the relay node non-hearing interval may meanthe interval in which a relay node transmits an access downlink signalinstead of receiving a backhaul downlink signal. As mentioned in theforegoing description, this relay node non-hearing interval may beconfigured to have 1-, 2- or 3-OFDM length. In the relay nodenon-hearing interval 1021, a relay node performs an access downlinktransmission to a user equipment and may receive a backhaul downlinkfrom a base station in the rest region 1022. In doing so, since therelay node is unable to perform both transmission and reception on thesame frequency band, it may take a time to enable the relay node to beswitched from a transmitting mode to a receiving mode. Hence, it may benecessary to configure a guard time (GT) to enable the relay node toperform a transmitting/receiving mode switching in first partialinterval of a backhaul downlink receiving region 1022. Similarly, evenif the relay node operates in a manner of receiving a backhaul downlinkfrom the base station and transmitting and access downlink to the userequipment, it may be able to configure a guard time (GT) for thetransmitting/receiving mode switching of the relay node. The length ofthe guard time may be defined as a value in time domain. For example,the length of the GT may be defined as k time samples (Ts) (where, k≧1)or may be set to the length of at least one or more OFDM symbols.Alternatively, in case that relay node backhaul downlink subframes arecontiguously configured or in accordance with a prescribed subframetiming alignment relation, the guard time of a last part of a subframemay be defined or may not configured. In order to maintain backwardcompatibility, this guard time may be defined only in a frequency domainconfigured for a backhaul downlink subframe transmission (i.e., a legacyuser equipment is not supportable if a guard time is configured in anaccess downlink interval). In the backhaul downlink receiving interval1022 except the guard time, the relay node may be able to receive relaynode dedicated PDCCH and PDSCH from the base station. In the meaning ofa relay node dedicated physical channel, the PDCCH and the PDSCH mayalso be represented as R-PDCCH (Relay-PDCCH) and R-PDSCH (Relay-PDSCH),respectively.

Inter-Cell Coordination & Transmission/Reception of Control Channel

In an advanced wireless communication system (e.g., LTE-A system, LTERelease-10 system, etc.), how to apply an enhanced inter-cellinterference coordination (eICIC) scheme to a case that an inter-cellinterference is greater than a signal of a serving cell in aheterogeneous network is currently discussed. The heterogeneous networkmay include a macro-pico case and/or a macro-femto case for example. Apico cell is able to exchange information with another cell (e.g., amacro cell) via X2 interface (or backhaul link) but a femto cell doesnot transceive information with another cell via the X2 interface.

For one example of various eICIC schemes, a method for aninterference-giving cell (i.e., an aggressor cell) not to perform atransmission in a specific resource region for a user equipmentconnected to an interference-given cell (i.e., a victim cell) [i.e.,represented as a null signal transmission or a silencing] may beapplicable. For example of a silencing operation, an aggressor cell canconfigure a specific subframe as an MBSFN subframe. In a DL subframeconfigured as an MBSFN subframe, a signal is transmitted in a controlregion only but is not transmitted in a data region. For another exampleof a silencing operation, an aggressor cell may configure a specificsubframe as ABS (almost blank subframe) or ABS-with-MBSFN. In this case,the ABS means a subframe having CRS transmitted in control and dataregions of a DL subframe only without transmitting control informationand data. Yet, in the ABS, a DL channel (e.g., PBCH (physical broadcastchannel), etc.) and a DL signal (e.g., PSS (primary synchronizationsignal), SSS (secondary synchronization signal, etc.) can betransmitted. The ABS-with-MBSFN means a case that CRS of a data regionis not transmitted in the above-mentioned ABS.

In this case, the silencing performed specific resource region can berepresented as a time resource and/or a frequency resource. Forinstance, a silenced time resource location may be determined by acombination of at least one of a whole time region, a specific subframe,a specific slot and a specific OFDM symbol unit. For instance, asilenced frequency resource location may be determined by a combinationof at least one of a whole frequency band, a specific carrier (in caseof carrier aggregation using a plurality of carriers), a specificresource block and a specific subcarrier unit. Therefore, a silencingperformed resource region can be clearly specified up to a resourceelement (RE) unit.

FIG. 11 is a diagram for one example of ABS application. In the exampleshown in FIG. 11, assume that inter-cell subframe boundary is identical.In particular, assume that a start timing of a subframe of an aggressorcell and a start timing of a subframe of a victim cell match each other.

In case that ABS is applied as part of eICIC, since all REs except CRSin the resource of an aggressor cell are configured as null REs, asshown in FIG. 11, interference is not caused to a victim cell by the REsexcept the CRS. Hence, inter-cell interference can be considerablyreduced. Moreover, FIG. 11 shows one example that CRS in one cell isshifted [V_(shift)] by 1 RE in frequency direction to prevent CRSlocations of the aggressor and victim cells from overlapping with eachother. If MBSFN can be configured for the aggressor cell, the generationof the interference caused by the CRS of the aggressor cell can bereduced in a data region. Hence, a user equipment of the victim cellconsiderably affected by the interference from the aggressor cell isable to maintain a communication with a serving cell (i.e., the victimcell) with good performance in a subframe set by the aggressor cell toABS and/or MBSFN subframe.

Meanwhile, PCFICH mapped resource elements are described with referenceto FIG. 12 as follows.

First of all, PCFICH among control channels of LTE or LTE-A system isthe channel that carries the information (i.e., control channel formatindicator (CCFI)) on the number of OFDM symbols used for a PDCCHtransmission in a single subframe. In this case, the number of the PFDMsymbols used for the PDCCH transmission may be set to 1, 2, or 3 forexample. The PCFICH contains 32-bit information for example, undergoescell-specific scrambling, QPSK modulation, layer mapping and precoding,and is then mapped to and carried on resource elements (REs). Fourcontiguous REs among the rest of REs except the RE used by a referencesignal (e.g., CRS) in a firstfirst OFDM symbol (i.e., OFDM symbol 0) ofa single subframe can be configured into one group (or a quadruplet). Inthe example shown in FIG. 12, REs denoted by 0, 1, 2 and 3 configure onegroup and REs denoted by 4, 5, 6 and 7 configure another group.Likewise, REs denoted by 8, 9, 10 and 11 configure one group and REsdenoted by 12, 13, 14 and 15 configure another group. The PCFICH can bethen mapped to the above-configured 4 groups. A start point (or anoffset) of the PCFICH mapped RE location, an interval between the groupsand the like can be determined depending on a system bandwidth, thenumber of REs configuring one RB, a cell identifier and the like.Therefore, the PCFICH can be transmitted at a specific RE location in afirstfirst OFDM symbol (i.e., OFDM symbol 0) of one subframe.

In the example shown in FIG. 11, in case that a subframe of an aggressorcell is configured as ABS, interference with a victim cell isconsiderably reduced. Yet, interference caused by CRS of the aggressorcell still exists, thereby causing a problem of performance degradationof the victim cell. Particularly, since the PCFICH of the victim cell istransmitted in a first OFDM of the subframe, it is directly affected bythe CRS from the aggressor cell.

FIG. 13 is a diagram to illustrate interference caused to PCFICH of avictim cell in case of applying such an eICIC scheme as shown in FIG.11. Like the example shown in FIG. 11, if a CRS location of an aggressorcell is shifted by 1 RE on a frequency axis in comparison with a CRSlocation of a victim cell, interference by the CRS of the aggressor cellis caused to 50% of PCFICH mapped REs on a first OFDM symbol of thevictim cell. In particular, referring to FIG. 13, in case that thePCFICH of the victim cell is transmitted on 16 REs, the REs denoted by0, 2, 4, 6, 8, 10, 12 and 14 among the 16 REs receive the interferencecaused by the CRS of the aggressor cell, a user equipment of the victimcell may not decode the PCFICH correctly.

If the user equipment is unable to decode the PCFICH correctly, it isnot able to be aware of the number of PDCCH transmitted OFCM symbols.Therefore, probability of failure in decoding PDCCH is raised andprobability of failure in decoding another control channel andprobability of failure in decoding a data channel are then considerablyraised as well. Thus, the PCFICH decoding failure is directly connectedto the degradation of overall system performance.

In the above-described heterogeneous network environment, in aspect ofthe victim cell, it may consider improving PCFICH decoding performanceof the victim cell by reducing the interference (i.e., CRS transmissionpower of the aggressor cell) caused by the CRS of the aggressor cell.Yet, in aspect of the aggressor cell, since the CRS needs to betransmitted with a transmission power higher than that of other signalsto be receivable by all user equipments in a cell, if the transmissionpower of the CRS is reduced, it may cause performance degradation of theaggressor cell. Therefore, the eICIC scheme by the transmission powercontrol is unable to solve the problem fundamentally.

The present invention proposes various examples of methods for solvingthe problem of the PCFICH decoding performance degradation mentioned inthe above description.

Embodiment 1

According to the present embodiment, a victim cell can differentiate amethod of determining a value of an OFDM symbol used for a transmissionof PDCCH depending on whether a DL subframe corresponds to a first typesubframe or a second type subframe. In this case, the first typesubframe may correspond to a coordinated subframe and the second typesubframe may correspond to an uncoordinated subframe.

In a coordinated subframe, a victim cell can use a predefined value N asthe number of OFDM symbols used for a PDCCH transmission. In this case,the coordinated subframe may mean the subframe of the victim cellcorresponding to a subframe configured as ABS, MBSFN or ABS-with-MBSFNby an aggressor cell. And, the number of the OFDM symbols used for thePDCCH transmission corresponds to a value (e.g., CCFI) transmitted onPCFICH. In particular, the present embodiment proposes that the numberof the OFDM symbols used for the PDCCH transmission in the coordinatedsubframe is provided not via the PCFICH but using the predefined valueN. In this case, the predefined value N may include one of 1, 2 and 3for example. Alternatively, the predefined value N may become 3 that isa maximum value of the number of the OFDM symbols used for the PDCCHtransmission.

The victim cell can signal the number of the OFDM symbols used for thePDCCH transmission via the PCFICH, like the existing method, for theuser equipments that receive relatively small influence of theinterference in the uncoordinated subframe. The user equipmentsreceiving relatively small influence of the interference may include auser equipment located at the center of the victim cell.

In particular, the victim cell uses the predefined value N as the numberof the OFDM symbols used for the PDCCH transmission in the coordinatedsubframe and can signal the number of the OFDM symbols used for thePDCCH transmission via the PCFICH in the uncoordinated subframe.

FIG. 14 is a flowchart to describe an operation of a base stationaccording to the present invention.

Referring to FIG. 14, a serving cell base station can determine whethera prescribed subframe is a coordinated subframe or an uncoordinatedsubframe [S1410].

If a result of the step S1410 is YES, the number of PDCCH transmissionOFDM symbols can be configured as a predefined value N [S1420]. Hence, aPDCCH mapped DL subframe can be transmitted on OFFDM symbols amountingto the number corresponding to the predefined value N [S1440].

On the contrary, if a result of the step S1410 is NO, the number ofPDCCH transmission OFDM symbols can be configured as a value to betransmitted on PCFICH [S1430]. Hence, a PDCCH mapped DL subframe can betransmitted on OFFDM symbols amounting to the number transmitted on thePCFICH [S1440].

Embodiment 2

As mentioned in the foregoing description of the embodiment 1, if aserving cell performs an operation of determining the number of PDCCHtransmission OFDM symbols in accordance with a predefined value or anoperation of providing the number of PDCCH transmission OFDM symbols viaPCFICH, a user equipment may not be aware how the serving cell operates.

If the serving cell is able to accurately inform the user equipment thata prescribed subframe is a coordinated subframe or an uncoordinatedsubframe, the user equipment can be aware that the number of PDCCHtransmission OFDM symbols in the coordinated subframe is determined as apredefined value N or that the number of PDCCH transmission OFDM symbolsin the uncoordinated subframe needs to be obtained from PCFICH.Therefore, the user equipment can perform a PDCCH decoding operationcorrectly.

Yet, it may happen that the serving cell is unable to inform the userequipment of a subframe configuration (i.e., whether the prescribedsubframe is the coordinated subframe or the uncoordinated subframe) inadvance. Despite that the serving cell has informed the user equipmentof the subframe configuration in advance, it may also happen that asubframe is configured different from the subframe configurationindicated by the serving cell. In this case, the user equipment may notbe able to determine whether to apply the predefined value as the numberof PDCCH transmission OFDM symbols or to attempt to obtain the valuethrough decoding of the PCFICH. To solve this problem, the presentembodiment proposes a method of determining an operation for PCFICHdecoding based on a value measured by a user equipment.

First of all, a user equipment measures and calculates a reception statefor a DL transmission from a neighboring cell and is then able to reportthe measured and calculated reception state. Subsequently, the servingcell can perform such an operation as a handover and the like based onthe DL reception state reported by the user equipment. In this case, themeasurement result indicating the reception state for the DLtransmission may include Reference Signal Received Power (RSRP),Reference Signal Received Quality (RSRQ), Received Signal StrengthIndicator (RSSI) and the like. Based on the values (e.g., RSRP) measuredby the user equipment, the user equipment can determine an operation forthe PCFICH decoding.

In particular, using RSRPs of the serving cell and other cell(s), theuser equipment is able to determine whether to apply a predeterminedvalue as the number of PDCCH transmission OFDM symbols or to find outthe number of PDCCH transmission OFDM symbols through the PCFICHdecoding. In the following description, the other cell(s) shall be nameda measured cell.

For instance, the user equipment can measure/calculate RSRP(RSRP_(ServingCell)) of the serving cell, RSRP (RSRP_(MeasuredCell)) ofthe measured cell, and a difference value(Diff_(RSRP)=RSRP_(ServingCell)−RSRP_(MeasuredCell)) in-between.Moreover, a prescribed reference value (Threshold_(PCFICH)) for the RSRPdifference value can be defined. Hence, the user equipment can determinethe operation for the PCFICH decoding depending on the relation betweenthe Diff_(RSRP) and the Threshold_(PCFICH).

In this case, the Threshold_(PCFICH) can be determined as a referencevalue determined relative to a previous absolute reference value (e.g.,a reference value related to the determination of handover execution).For example, a value higher than the previous absolute reference valueby a prescribed size may be named Threshold_(PCFICH,HIGH). And, a valuelower than the previous absolute reference value by a prescribed sizemay be named Threshold_(PCFICH,LOW).

The Threshold_(PCFICH,HIGH) may be used as a reference value fordetermining an operation for the PCFICH decoding if the user equipmentis unable to receive a handover command from the serving cell despite asituation in which a signal from the serving cell is stronger than thatof the other cell (i.e., the measured cell). For instance, ifDiff_(RSRP)>Threshold_(PCFICH,HIGH), the user equipment attempts toobtain the number of PDCCH transmission OFDM symbols through the PCFICHdecoding. If Diff_(RSRP)≦Threshold_(PCFICH,HIGH), the user equipment mayoperate to apply a predetermined value to the number of PDCCHtransmission OFDM symbols.

The Threshold_(PCFICH,LOW) may be used as a reference value fordetermining an operation for the PCFICH decoding if the user equipmentreceives a handover command from the serving cell despite a situation inwhich a signal from the serving cell is stronger than that of the othercell (i.e., the measured cell). For instance, ifDiff_(RSRP)>Threshold_(PCFICH,LOW), the user equipment attempts toobtain the number of PDCCH transmission OFDM symbols through the PCFICHdecoding. If Diff_(RSRP)≦Threshold_(PCFICH,LOW), the user equipment mayoperate to apply a predetermined value to the number of PDCCHtransmission OFDM symbols.

In addition, the Threshold_(PCFICH) may be determined based on PCFICHperformance. In particular, the Threshold_(PCFICH) can be determined tocorrespond to a minimum SINR (Signal-to-Interference plus Noise Ratio)at which the PCFICH can be correctly decoded. In more particular, anRSRP difference value, which does not meet the decoding condition of thePCFICH because the RSRP of the interference-giving cell is greater thanthe RSRP of the interference-given cell, can be determined as theThreshold_(PCFICH).

FIG. 15 is a flowchart to describe an operation of a user equipmentaccording to the present invention.

Referring to FIG. 15, a user equipment measures RSRP(RSRP_(ServingCell)) of a serving cell and RSRP (RSRP_(MeasuredCell)) ofa neighboring cell [S1510] and is then able to calculate a differencevalue Diff_(RSRP) (=RSRP_(ServingCell)−RSRP_(MeasuredCell)) in-between[S1520].

The user equipment compares a predetermined reference valueThreshold_(PCFICH)) to the Diff_(RSRP) calculated in the step S1520,thereby determining whether it is Diff_(RSRP)≦Threshold_(PCFICH)[S1530]. If a value of the Diff_(RSRP) gets higher, it corresponds to acase that interference with the neighboring cell becomes smaller. If thevalue of the Diff_(RSRP) gets lower (e.g., a negative number included),it corresponds to a case that the interference with the neighboring cellbecomes bigger. Hence, if Diff_(RSRP)>Threshold_(PCFICH), theinterference with the neighboring cell may correspond to a level notenough to degrade the PCFICH decoding performance. IfDiff_(RSRP)≦Threshold_(PCFICH), the interference with the neighboringcell may correspond to a level enough to degrade the PCFICH decodingperformance.

If the result of the step S1530 is YES, it may correspond to a case thatthe interference with the neighboring cell is higher than the reference.In this case, the user equipment can assume that PDCCH is mapped to thenumber of OFDM symbols amounting to a predefined number N [S1540].Subsequently, the user equipment can perform the PDCCH decoding based onthe assumption of the step S1540 [S1560].

If the result of the step S1530 is NO, it may correspond to a case thatthe interference with the neighboring cell is lower than the reference.In this case, the user equipment can obtain the number of PDCCHtransmission OFDM symbols from PCFICH [S1550]. Subsequently, the userequipment can perform the PDCCH decoding based on the number of PDCCHtransmission OFDM symbols obtained in the step S1550 [S1560].

Embodiment 3

FIG. 16 is a diagram to describe one embodiment of the present inventionin a heterogeneous network between a macro cell and a pico cell. Inparticular, FIG. 16 shows a case that cell range expansion (CRE) isapplied in a pico cell. In this case, the CRE is a scheme of increasingoverall system throughput by reducing a load put on a macro cell byforcing a range of the pico cell to expand. In this case, a method for auser equipment (PUE), which is served by the pico cell, to avoid/reduceinterference from a macro cell is required.

In the example shown in FIG. 16, the PUE is moving away into a rangeexpansion area from a center location of the pico cell. Thus, while thePUE is moving, RSRP of a serving cell (i.e., the pico cell) graduallydecreases but RSRP of a neighboring cell (i.e., the macro cell)gradually increases. Hence, Diff_(RSRP)(=RSRP_(ServingCell)−RSRP_(MeasuredCell)) gradually decreases. In thiscase, it becomes highly probable that the user equipment is unable tocorrectly decode PCFICH from the pico cell due to the stronginterference (particularly, interference caused by CRS of the macrocell) from the macro cell. In this case, Threshold_(PCFICH,HIGH) may bedetermined as a prescribed reference value to determine whether the userequipment uses a predefined value N as the number of PDCCH transmissionOFDM symbols or obtains a value through the PCFICH. And, theThreshold_(PCFICH,HIGH) can be determined as the value higher than anRSRP reference value for a handover.

As a user equipment gets away from a center of a pico cell, PCFICHdecoding performance is lowered. Yet, according to the presentinvention, it is able to solve a problem that the user equipment isunable to correctly decode PCFICH as well as to make a handover into amacro cell due to failing in receiving a signaled handover message. Inparticular, if it is Diff_(RSRP)≦Threshold_(PCFICH,HIGH) and thehandover message is not signaled, the user equipment assumes that thenumber of PDCCH transmission OFDM symbols is a predefined value N and isthen able to perform PDCCH decoding correspondingly. Alternatively, whenan absolute value (|Diff_(RSRP)|) of the Diff_(RSRP) increases graduallyand then becomes equal to or greater than an absolute value(|Threshold_(PCFICH,HIGH)|) of the Threshold_(PCFICH,HIGH), if thehandover message is not signaled, the user equipment assumes that thenumber of PDCCH transmission OFDM symbols is a predefined value N and isthen able to operate to perform PDCCH decoding correspondingly. In thiscase, when the operation of the user equipment is determined by thecomparison between |Diff_(RSRP)| and |Threshold_(PCFICH,HIGH)|, it mayexclude a case that the user equipment is facilitated to performdecoding of the PCFICH from the serving cell because RSRP_(Servingcell)is greater than RSRP_(MeasuredCell).

For instance, in case that the CRE is not applied, when the userequipment gets out of a range of the pico cell and then enters a rangeof the macro cell, the handover message should be signaled from theserving cell (i.e., the pico cell). Yet, as the CRE is applied, a powerof the signal from the pico cell is weakened and interference with themacro cell is considerably strong. In doing so, if the handover messageis not signaled from the serving cell (i.e., the pico cell), theproposal of the present invention mentioned in the foregoing descriptioncan be advantageously applied.

Moreover, although the above-described example limitedly relates to thecase that the handover message is not received, the user equipment candetermine whether to perform the PCFICH decoding operation irrespectiveof a presence or non-presence of the handover message signaling if thePCFICH decoding performance is lowered. For instance, if a calculationresult of Diff_(RSRP) is determined enough for RSRP_(MeasuredCell) toaffect the PCFICH decoding performance (this can be determined by thecomparison between Diff_(RSRP) and Threshold_(PCFICH,HIGH) or acomparison between Diff_(RSRP) and Threshold_(PCFICH)), the userequipment can perform the PDCCH decoding by assuming that the number ofPDCCH transmission OFDM symbols is a predefined value N (i.e., withoutthe PCFICH decoding operation). For instance, if it isDiff_(RSRP)≦Threshold_(PCFICH,HIGH), the user equipment reports theRSRP_(MeasuredCell) to the serving cell (i.e., the pico cell), receivesacknowledgement (i.e., ACK message) from the serving cell, and is thenable to directly perform the PDCCH decoding by assuming that the numberof PDCCH transmission OFDM symbols is the predefined value N (i.e.,without the PCFICH decoding operation).

Embodiment 4

FIG. 17 is a diagram to describe another embodiment of the presentinvention in a heterogeneous network between a macro cell and a picocell. In particular, FIG. 16 shows a case that cell range expansion(CRE) is applied in a pico cell.

One example of applying the present invention to a case that a userequipment (MUE) served by a macro cell moves into a range expansion areaof a pico cell from a center of the macro cell is described withreference to FIG. 17 as follows.

Thus, while the MUE is moving, RSRP of a serving cell (i.e., the macrocell) gradually decreases but RSRP of a neighboring cell (i.e., the picocell) gradually increases. Hence, Diff_(RSRP)(=RSRP_(ServingCell)−RSRP_(MeasuredCell)) gradually decreases. In thiscase, it becomes highly probable that the user equipment is unable tocorrectly decode PCFICH from the macro cell due to the stronginterference (particularly, interference caused by CRS of the pico cell)from the pico cell. In this case, Threshold_(PCFICH,LOW) may bedetermined as a prescribed reference value to determine whether the userequipment uses a predefined value N as the number of PDCCH transmissionOFDM symbols or obtains a value through the PCFICH. And, theThreshold_(PCFICH,LOW) can be determined as the value lower than an RSRPvalue for a handover.

According to the present invention, as the user equipment gets away fromthe center of the macro cell and then enters the range expansion area ofthe pico cell, it may happen that the user equipment receives a singlingof a message indicating a handover from the macro cell into the picocell despite that the RSRP from the macro cell is still higher than theRSRP from the pico cell. In this case, before the user equipment makesthe handover, if it is Diff_(RSRP)≦Threshold_(PCFICH,LOW) and thehandover message is signaled, the user equipment assumes that the numberof PDCCH transmission OFDM symbols is a predefined value N and is thenable to perform PDCCH decoding correspondingly. Alternatively, when anabsolute value (|Diff_(RSRP)|) of the Diff_(RSRP) increases graduallyand then becomes equal to or smaller than an absolute value(|Threshold_(PCFICH,LOW)|) of the Threshold_(PCFICH,HIGH), if thehandover message is signaled, the user equipment assumes that the numberof PDCCH transmission OFDM symbols is a predefined value N and is thenable to operate to perform PDCCH decoding correspondingly. Inparticular, the user equipment is able to determine the number of PDCCHtransmission OFDM symbols without decoding the PCFICH.

For instance, in case that the CRE is not applied, when the userequipment gets out of a range of the macro cell and then enters a rangeof the pico cell, the handover message should be signaled from theserving cell (i.e., the macro cell). Yet, as the CRE is applied,although a power of the signal from the macro cell is still strong andinterference with the pico cell is not strong, if the handover messageis signaled from the serving cell (i.e., the macro cell), the proposalof the present invention mentioned in the foregoing description can beadvantageously applied.

Moreover, although the above-described example limitedly relates to thecase that the handover message is received, the user equipment candetermine whether to perform the PCFICH decoding operation irrespectiveof a presence or non-presence of the handover message signaling if thePCFICH decoding performance is lowered. For instance, if a calculationresult of Diff_(RSRP) is determined enough for RSRP_(MeasuredCell) toaffect the PCFICH decoding performance (this can be determined by thecomparison between Diff_(RSRP) and Threshold_(PCFICH,LOW) or acomparison between Diff_(RSRP) and Threshold_(PCFICH)), the userequipment can perform the PDCCH decoding by assuming that the number ofPDCCH transmission OFDM symbols is a predefined value N (i.e., withoutthe PCFICH decoding operation). For instance, if it is Diff_(RSRP)Threshold_(PCFICH,LOW), the user equipment reports theRSRP_(MeasuredCell) to the serving cell (i.e., the macro cell), receivesacknowledgement (i.e., ACK message) from the serving cell, and is thenable to directly perform the PDCCH decoding by assuming that the numberof PDCCH transmission OFDM symbols is the predefined value N (i.e.,without the PCFICH decoding operation).

Moreover, after a user equipment has made a handover, a serving cellbecomes a pico cell and a neighboring cell becomes a macro cell. Indoing so, in case that the user equipment is located in a rangeexpansion area of the pico cell, the present invention can be applied.In this case, if DIFF_(RSRP) measured by the user equipment is enough toaffect PCFICH decoding performance due to strong interference with themacro cell (i.e., Diff_(RSRP)≦Threshold_(PCFICH)), the user equipment isable to perform the PDCCH decoding by assuming that the number of PDCCHtransmission OFDM symbols is the predefined value N without the PCFICHdecoding operation.

Embodiment 5

FIG. 18 is a diagram to describe one embodiment of the present inventionin a heterogeneous network between a macro cell and a femto cell. InFIG. 18, assume that the femto cell is a cell of a CSG (closedsubscriber group) type. And, assume a case that a user equipment (UE)having no authority in using a femto cell is not able to access thecorresponding femto cell. Such a femto cell may include a home basestation (Home eNB (HeNB)). And, this femto cell does not exchangeinformation with a macro cell via X2 interface.

In the example shown in FIG. 18, assume that an MUE includes a userequipment having no authority in using a femto cell. And, assume thatthe MUE is served by a macro cell. In case that the MUE is locatedwithin an area of the femto cell, the MUE receives strong interferencefrom the femto cell. And, the femto cell may perform an eICIC operationof configuring at least one of DL subframes as ABS and/or MBSFN. In thiscase, the principle of the present invention may be applicable as amethod for the MUE located within the area of the femto cell todetermine whether to decode PCFICH from the macro base station (macroeNB).

In particular, since the user equipment does not have the authority inusing the femto cell having a high signal power, the user equipment isnot able to access the femto cell but has to access the macro cellhaving a low signal power. In doing so, if a value Diff_(RSRP)(=RSRP_(ServingCell)−RSRP_(MeasuredCell)) resulting from subtractingRSRP (RSRP_(MeasuredCell)) of the femto cell from RSRP(RSRP_(ServingCell)) of the macro cell is equal to or smaller than aprescribed reference value (Threshold_(PCFICH)), the user equipmentassumes that the number of PDCCH transmission OFDM symbols from aserving cell (i.e., the macro cell) is a predefined value N and is thenable to perform PDCCH decoding correspondingly. Alternatively, if anabsolute value (|Diff_(RSRP)|) of the Diff_(R) becomes equal to orgreater than an absolute value (|Threshold_(PCFICH,LOW)|) of theThreshold_(PCFICH,LOW), the user equipment assumes that the number ofPDCCH transmission OFDM symbols is a predefined value N and is then ableto operate to perform PDCCH decoding correspondingly. In particular, theuser equipment can determine the number of PDCCH transmission OFDMsymbols without decoding the PCFICH.

In this case, the Threshold_(PCFICH) may be determined as a valuecorresponding to a minimum SINR for decoding the PCFICH correctly. Inparticular, a minimum value of the RSRP difference value, which does notmeet the decoding condition of the PCFICH because the RSRP of aninterference-giving cell is greater than that of an interference-givencell, can be determined as the Threshold_(PCFICH).

Embodiment 6

The present embodiment relates to an example of applying the presentinvention to a case that the user equipment in the example shown in FIG.18 performs an initial access process.

In FIG. 18, it is able to assume a case that the user equipmentunauthorized in accessing the femto cell attempts an initial accesswithin the area of the femto cell. In this case, the user equipmentperforms a random access process to access the macro cell. In order toperform the random access process, the user equipment can obtaininformation required for RACH transmission, system information and thelike by decoding a system information block (SIB) transmitted from themacro base station prior to the RACH (random access channel)transmission. The user equipment starts the RACH transmission to themacro base station using the information obtained from decoding the SIBand the macro base station can transmit a response message (RACHresponse) to the user equipment in response to the RACH transmitted bythe user equipment. The SIB and the RACH response are transmitted onPDSCH to the user equipment. Moreover, locations of the SIB and the RACHresponse within the PDSCH can be obtained by decoding a control channel(e.g., PCFICH, PDCCH, etc.) only.

In doing so, since the user equipment located within the area of thefemto cell is receiving strong interference from the femto cell, it isunlikely to happen that the user equipment decodes the PCFICH from themacro base station. If the user equipment fails in decoding the PCFICH,the probability for the user equipment to receive the SIB and the RACHresponse on the PDSCH in a manner of obtaining the number of PDCCHtransmission OFDM symbols and then decoding the PDCCH based on theobtained number of PDCCH transmission OFDM symbols is considerablylowered. To solve this problem, it is able to apply the principle of thepresent invention thereto.

In the aforementioned initial access process performed by the userequipment, the user equipment can measure RSRPs of neighboring cells. Inaccordance with a result of the measurement, it may happen that a signalquality of a cell the user equipment intends to access receives a stronginterference with a signal of the neighboring cell(s). For instance, ifthe user equipment does not belong to the CSG of the femto cell, theuser equipment is unable to access the femto cell despite that the RSRPfrom the femto cell is strong. Hence, a case that the user equipmentattempts to access a different cell (e.g., macro cell) may correspond tothis situation. In this case, if a difference between the RSRP of thecell (macro cell) the user equipment attempts to access and the RSRP ofthe interference-causing cell (femto cell) is greater than a prescribedreference value (e.g., Threshold_(PCFICH)), the user equipment assumesthat the number of PDCCH transmission OFDM symbols is a predefined valueN and is then able to attempt the PDCCH decoding without decoding thePCFICH. In doing so, the macro cell can perform a PDCCH transmissionusing OFDM symbols amounting the same number of the predefined value N.Alternatively, in case that the SIB or the RACH response is transmittedfrom the macro cell, the number of PDCCH transmission OFDM symbols maybe set equal to the predefined value N.

Accordingly, the user equipment finds out location information of theSIB by decoding the PDCCH and is then able to decode the correspondingSIB. Subsequently, the user equipment can transmit RACH to the macrocell using the information obtained from the SIB. In case that the userequipment receives the RACH response from the macro cell, the userequipment finds out a location of the RACH response by decoding thePDCCH without decoding the PCFICH and is then able to correctly decodethe RACH response.

As mentioned in the foregoing descriptions of various embodiments of thepresent invention, a method for a user equipment to determine the numberof PDCCH transmission OFDM symbols can be advantageously applied to acase of performing inter-cell interference coordination, an initialaccess operation of a user equipment and the like.

In particular, according to the present invention, in a PDCCH receivingoperation of a user equipment, a received signal power of a first cell(e.g., a serving cell, a victim cell, a cell to be initially accessed bya user equipment, etc.) and a received signal power of a second cell(e.g., a neighboring cell, an aggressor cell, a cell causinginterference to a user equipment except a cell to be initially accessedby a user equipment, etc.) are measured by the user equipment and adifference value between the measured received signal powers of therespective cells can be compared and determined. Depending on a resultof the determination, the user equipment can determine whether toperform PDCCH decoding on the assumption that the number of PDCCHtransmission OFDM symbols is a predefined value N or to perform PDCCHdecoding by obtaining the number of PDCCH transmission OFDM symbols in amanner of attempting PCFICH decoding.

The contents and/or items explained in the descriptions of the variousembodiments of the present invention may be independently applicable orat least two embodiments of the present invention may be simultaneouslyapplicable. And, redundant descriptions shall be omitted from thefollowing description for clarity.

Although the operations in the base station (cell) and the userequipment are exemplarily described for clarity in the aforementionedvarious embodiments of the present invention, the description of theoperation in the base station (cell) is identically applicable to anoperation in a relay node device as a downlink transmitting entity or an uplink receiving entity and the description of the operation in theuser equipment is identically applicable to a relay node device as anuplink transmitting entity or a downlink receiving entity.

FIG. 19 is a diagram for configurations of a base station device 1910and a user equipment device 1920 according to one preferred embodimentof the present invention.

Referring to FIG. 19, a base station device 1910 according to oneembodiment of the present invention may include a receiving module 1911,a transmitting module 1912, a processor 1913, a memory 1914 and aplurality of antennas 1915. A plurality of the antennas 1915 may mean abase station device supportive of MIMO transmission and reception. Thereceiving module 1911 can receive various signals, data and informationin uplink from a user equipment. The transmitting module 1912 cantransmit various signals, data and information in downlink to the userequipment. And, the processor 1913 can control overall operations of thebase station device 1910.

The base station device 1910 according to one embodiment of the presentinvention can be configured to transmit a DL control channel. Theprocessor 1913 of the base station device 1910 can be configured todetermine whether a DL subframe corresponds to a first type subframe ora second type subframe. The processor 1913 sets the number of OFDMsymbols used for a transmission of the DL control channel (e.g., PDCCH)to a predefined number N if the DL subframe is the first type subframe.And, the processor 193 can be configured to transmit the DL controlchannel on the OFDM symbols amounting to the number equal to the N viathe transmitting module 1912. If the DL subframe is the second typesubframe, the processor 1913 transmits information on the number of OFDMsymbols used for a transmission of the DL control channel via PCFICH andcan be configured to transmit the DL control channel on the OFDM symbolsamounting to the same number indicated by the information transmitted onthe PCFICH.

The first type subframe may include a subframe on which inter-cellinterference coordination is performed by a neighboring cell. And, thesecond type subframe may correspond to a subframe on which theinter-cell interference coordination is not performed by the neighboringcell. Moreover, the inter-cell interference coordination performedsubframe may correspond to a subframe configured as ABS, MBSFN subframeor ABS-with-MBSFN subframe by the neighboring cell.

In this case, in the DL subframe corresponding to the first typesubframe, the information on the number of the OFDM symbols used for thetransmission of the DL control channel may not be transmitted. Inparticular, the base station may not provide the number of PDCCHtransmission OFDM symbols via the PCFICH.

The processor 1913 of the base station device 1910 performs functions ofoperating and processing information received by the base station device1910, information to be transmitted by the base station device 1910 andthe like. The memory 1914 can store the operated and processedinformation and the like for a prescribed period and can be substitutedwith such a component as a buffer (not shown in the drawing) and thelike.

Referring to FIG. 19, a user equipment device 1920 according to oneembodiment of the present invention may include a receiving module 1921,a transmitting module 1922, a processor 1923, a memory 1924 and aplurality of antennas 1925. A plurality of the antennas 1925 may mean auser equipment device supportive of MIMO transmission and reception. Thereceiving module 1921 can receive various signals, data and informationin downlink from a base station. The transmitting module 1922 cantransmit various signals, data and information in uplink to the basestation. And, the processor 1923 can control overall operations of theuser equipment device 1920.

The user equipment device 1920 according to one embodiment of thepresent invention can be configured to receive a DL control signal. Theprocessor 1923 of the user equipment device 1920 can be configured tomeasure a power of a signal transmitted each of a serving cell and aneighboring cell. The processor 1923 can be configured to calculate adifference value (Diff_(RSRP)=RSRP_(ServingCell)−RSRP_(MeasuredCell))resulting from subtracting a power (RSRP_(MeasuredCell)) of a signalreceived from the neighboring cell from a power (RSRP_(ServingCell)) ofa signal received from the serving cell. The processor 1923 may beconfigured to compare the difference value (Diff_(RSRP)) to a prescribedreference value (Threshold_(PCFICH)). If the difference value(Diff_(RSRP)) is equal to or smaller than the prescribed reference value(Threshold_(PCFICH)), the processor 1923 assumes that the number of OFDMsymbols used for a transmission of a DL control channel (e.g., PDCCH)from the serving cell as a predefined value N and decode the DL controlchannel on the OFDM symbols amounting to a number equal to the N. if thedifference value (Diff_(RSRP)) is greater than the prescribed referencevalue (Threshold_(PCFICH)), the processor 1923 obtains the number of theOFDM symbols used for the transmission of the DL channel by decoding thePCFICH from the serving cell and is then able to decode the DL controlchannel on the OFDM symbols amounting to the number equal to theinformation obtained from the PCFICH.

In this case, the prescribed reference value (Threshold_(PCFICH)) can bedetermined based on an SINR value that meets a decoding condition of thePCFICH from the serving cell. Alternatively, the prescribed referencevalue (Threshold_(PCFICH)) can be determined based on a reception powerdifference for determining whether to operate a handover. For instance,the prescribed reference value may be set to a value(Threshold_(PCFICH,HIGH)) higher than the reception power differencevalue for the determining whether to operate the handover into theneighboring cell from the serving cell in a range expansion area of theserving cell or a value (Threshold_(PCFICH,LOW)) lower than thereception power difference value for determining whether to operate thehandover into the neighboring cell from the serving cell in a rageexpansion area of the neighboring cell.

And, the result of the measurement of the power of the signal receivedfrom each of the serving cell and the neighboring cell can be reportedto the base station.

The processor 1923 of the user equipment device 1920 performs functionsof operating and processing information received by the user equipmentdevice 1920, information to be transmitted by the user equipment device1920 and the like. The memory 1924 can store the operated and processedinformation and the like for a prescribed period and can be substitutedwith such a component as a buffer (not shown in the drawing) and thelike.

In the above-mentioned detailed configurations of the base stationdevice 1910 and the user equipment device 1920, the contents or itemsexplained in the descriptions of the various embodiments of the presentinvention may be independently applicable or at least two embodiments ofthe present invention may be simultaneously applicable. And, redundantdescriptions shall be omitted from the following description forclarity.

The description of the base station device 1910 with reference to FIG.19 may be identically applicable to a relay node device as a DLtransmitting entity or a UL receiving entity. And, the description ofthe user equipment device 1920 with reference to FIG. 19 may beidentically applicable to a relay node device as a UL transmittingentity or a DL receiving entity.

Embodiments of the present invention can be implemented using variousmeans. For instance, embodiments of the present invention can beimplemented using hardware, firmware, software and/or any combinationsthereof.

In case of the implementation by hardware, a method according to eachembodiment of the present invention can be implemented by at least oneselected from the group consisting of ASICs (application specificintegrated circuits), DSPs (digital signal processors), DSPDs (digitalsignal processing devices), PLDs (programmable logic devices), FPGAs(field programmable gate arrays), processor, controller,microcontroller, microprocessor and the like.

In case of the implementation by firmware or software, a methodaccording to each embodiment of the present invention can be implementedby modules, procedures, and/or functions for performing theabove-explained functions or operations. Software code is stored in amemory unit and is then drivable by a processor. The memory unit isprovided within or outside the processor to exchange data with theprocessor through the various means known to the public.

As mentioned in the foregoing description, the detailed descriptions forthe preferred embodiments of the present invention are provided to beimplemented by those skilled in the art. While the present invention hasbeen described and illustrated herein with reference to the preferredembodiments thereof, it will be apparent to those skilled in the artthat various modifications and variations can be made therein withoutdeparting from the spirit and scope of the invention. Thus, it isintended that the present invention covers the modifications andvariations of this invention that come within the scope of the appendedclaims and their equivalents. For instance, the respectiveconfigurations disclosed in the aforesaid embodiments of the presentinvention can be used by those skilled in the art in a manner of beingcombined with one another. Therefore, the present invention isnon-limited by the embodiments disclosed herein but intends to give abroadest scope matching the principles and new features disclosedherein.

While the present invention has been described and illustrated hereinwith reference to the preferred embodiments thereof, it will be apparentto those skilled in the art that various modifications and variationscan be made therein without departing from the spirit and scope of theinvention. Thus, it is intended that the present invention covers themodifications and variations of this invention that come within thescope of the appended claims and their equivalents. And, it isapparently understandable that an embodiment is configured by combiningclaims failing to have relation of explicit citation in the appendedclaims together or can be included as new claims by amendment afterfiling an application.

The embodiments of the present invention mentioned in the foregoingdescription are applicable to various kinds of mobile communicationsystems.

What is claimed is:
 1. A method of transmitting a downlink controlchannel to a user equipment, the method performed by a base station andcomprising: setting a number of OFDM (orthogonal frequency divisionmultiplexing) symbols used for transmission of the downlink controlchannel based on a predefined value if a downlink subframe correspondsto a first type subframe; transmitting information related to the numberof OFDM symbols used for transmission of the downlink control channelvia a PCFICH (physical control format indicator channel) if the downlinksubframe corresponds to a second type subframe; and transmitting thedownlink control channel on the OFDM symbols.
 2. The method of claim 1,wherein the predefined value is indicated by higher layer signaling. 3.The method of claim 1, wherein the downlink control channel comprises atleast a PDCCH (physical downlink control channel) or an E-PDCCH(Enhanced-PDCCH).
 4. The method of claim 1, wherein: the first typesubframe is a subframe on which an inter-cell interference coordinationis performed by a neighboring cell; and the second type subframe is asubframe on which no inter-cell interference coordination is performedby the neighboring cell.
 5. The method of claim 4, wherein the firsttype subframe is configured by the neighboring cell as an ABS (almostblank subframe), an MBSFN (multicast broadcast single frequency network)subframe, or an ABS-with-MBSFN subframe.
 6. A base station (BS) fortransmitting a downlink control channel to a user equipment (UE), the BScomprising: a receiving module configured to receive an uplink signalfrom the UE; a transmitting module configured to transmit a downlinksignal to the UE; and a processor configured to: set a number of OFDM(orthogonal frequency division multiplexing) symbols used for atransmission of the downlink control channel based on a predefined valueif a downlink subframe corresponds to a first type subframe; transmitinformation related to the number of OFDM symbols used for transmissionof the downlink control channel via a PCFICH (physical control formatindicator channel) if the downlink subframe corresponds to a second typesubframe; and transmit the downlink control channel on the OFDM symbols.7. The BS of claim 6, wherein the predefined value is indicated byhigher layer signaling.
 8. The BS of claim 6, wherein the downlinkcontrol channel comprises at least a PDCCH (physical downlink controlchannel) or an E-PDCCH (Enhanced-PDCCH).
 9. The BS of claim 6, wherein:the first type subframe is a subframe on which an inter-cellinterference coordination is performed by a neighboring cell; and thesecond type subframe is a subframe on which no inter-cell interferencecoordination is performed by the neighboring cell.
 10. The BS of claim9, wherein the first type subframe is a subframe configured by theneighboring cell as an ABS (almost blank subframe), an MBSFN (multicastbroadcast single frequency network) subframe, or an ABS-with-MBSFNsubframe.